If your game natively supports local co-op (a.k.a. "couch co-op") or split-screen, you should be able to play with just one instance of the game, supporting multiple inputs.

If it doesn't...

I don't think you can run more than one instance of Steam on a single computer, at one time, though I'm not entirely sure about this. On top of that, games don't necessarily let themselves run more than one instance at a time.

If you just want to run two instances of a game at the same time, then (1) it's probably better to get the games DRM-free if possible, so you don't need to deal with two different sets of permissions to run the game, and then (2) you need to make sure the game can run two simultaneous instances at once, or otherwise modify it so that it can.

On top of this, you'll need some way for the operating system to hold two applications in focus at the same time, or at least accept inputs to both. Operating systems are generally built to only have one application in focus at once, and while some applications will accept input even when out of focus, many applications are built so that they avoid this, so as to avoid funny things happening. (Some emulators may let this happen though.) So this is another thing you may need to tweak to get the game to work right.

(FWIW I know that someone has managed to run four instances of 100% Orange Juice, a multiplayer game with no local co-op option, on one computer at the same time, with any two of them in focus at any given time.)

Your only option would be one PC, one account and split screen games.
You can have multiple accounts per PC, that's about it.
It's designed more that way.
https://steamhost.cn/tags/en/Split%20Screen/

Yeah, as others point out, your best and far easiest result is looking for games that have local co-op built in - sadly they are far rarer on PC games than console, for the obvious reasons (even on the very same games).

There might be ways to wrestle to do what you wish but that's the term there - wrestle. You're going to have to fiddle with each game indivually and spend shed loads of time on messing around.

First of all: thanks a lot for taking your time to answer, I really appreciate it :)

Postat inițial de The_Unknown_05:

if you are a newbie in tech too I would say no

I'm a newbie at using the Steam platform, I used to buy physical games instead of digital versions.

Postat inițial de Quint the Alligator Snapper:

I don't think you can run more than one instance of Steam on a single computer, at one time, though I'm not entirely sure about this. On top of that, games don't necessarily let themselves run more than one instance at a time.

For what I tried, you can run the Steam software in one account and the web version of Steam in another one. You can even interact between them by chatting or playing with the software version and inviting the web one to watch you while playing. This is the farthest I've tried, though.

About running more than one instance at a time, I don't know about that either... I was guessing (wishing) only one would be needed to sum us both inside there from our respective accounts. But it is starting to sound like a big mess :P

@d𐂃 𐂃b & @crunchyfrog <-- So I understand that games like Trine, Trine 2/3/4 (coop. coop split screen) could be played by 2 people even using only one account, did I get it right?

Would it be possible though, playing coop Left 4 Dead 2 or Borderlands, using 2 accounts but one pc? That's the question that haunts us!

Postat inițial de Meryhop:

First of all: thanks a lot for taking your time to answer, I really appreciate it :)

Postat inițial de The_Unknown_05:

if you are a newbie in tech too I would say no

I'm a newbie at using the Steam platform, I used to buy physical games instead of digital versions.

Postat inițial de Quint the Alligator Snapper:

I don't think you can run more than one instance of Steam on a single computer, at one time, though I'm not entirely sure about this. On top of that, games don't necessarily let themselves run more than one instance at a time.

For what I tried, you can run the Steam software in one account and the web version of Steam in another one. You can even interact between them by chatting or playing with the software version and inviting the web one to watch you while playing. This is the farthest I've tried, though.

About running more than one instance at a time, I don't know about that either... I was guessing (wishing) only one would be needed to sum us both inside there from our respective accounts. But it is starting to sound like a big mess :P

@d𐂃 𐂃b & @crunchyfrog <-- So I understand that games like Trine, Trine 2/3/4 (coop. coop split screen) could be played by 2 people even using only one account, did I get it right?

Would it be possible though, playing coop Left 4 Dead 2 or Borderlands, using 2 accounts but one pc? That's the question that haunts us!

This. Steam must implement multi account login. Then more devs could implement split screen or hot seat modes in PC and not only in consoles. Please devs more local multiplayer!

With a game that supports split screen you would only need the one account running the game.

If the games support Remote Play Together, do that.

Postat inițial de cSg|mc-Hotsauce:

If the games support Remote Play Together, do that.

:qr:

I think the main problem, as I understand the thread, is about playing on one machine. Otherwise they could just LAN play.

Virtual machine.

Seems like what's being asked for is some kind of guest login system much like the consoles have. Something that can track playtime, achievements, stats, and also allow the guest to use any unlocked or purchased cosmetics/items. This would honestly be pretty cool, but of course devs would need to add it on their end as well if Valve ever added it to the Steamworks API.

i do the same

Postat inițial de Gambit-3k:

Seems like what's being asked for is some kind of guest login system much like the consoles have. Something that can track playtime, achievements, stats, and also allow the guest to use any unlocked or purchased cosmetics/items. This would honestly be pretty cool, but of course devs would need to add it on their end as well if Valve ever added it to the Steamworks API.

Exactly this, please.
Used to doing this on Xbox console, where my wife and I both may sign in to our own Xbox accounts on the one console and play local and online co-op in games that support split/shared screen gaming and both earn progress on achievements, etc.

There are some games where only the main profile will earn progress on some achievements. Overcooked for example we had to play twice in order to unlock all the achievements on both our accounts.