Can i copy game from one pc to another to play multiplayer games
And how do i know which games can be played on on pc with more than one player

You need two legit copies of a game to do this. Also, to tell if you can do it is easy. It shoudl say in the README file wether it does IPX or TCP/IP. Either one can be done on a home network.

your going to have fun if its IPX.... Ahh DOS.... how i loved thee!

I'd say that a multi-player rather qualifies as sharing with others, unless the original poster intends to play a multi-player match with himself ;)
It's less of a technical issue with GoG's DRM free attempt, but more of a moral one.

You are aware that A) not all people live alone, and B) some people have friends who occasionally visit them for the specific purpose of playing games, right? I'm pretty sure that "sharing with others" in this context means "giving copies of the game to others not in your household".

Screw IPX, Warcraft 2 before the Battle.net edition...:(

I am indeed fully aware of this. But while I'm certainly not in the position to even only guess what GoG had in mind when developing their concept, I can tell you that most every game publisher out there considers letting others play your copy on your computer fair game, but requests you to purchase an additional copy once more than one person is playing the game on more than one computer at the same time.
For example, two people playing hot seat would be fine with just one copy of the game, because they play sequential. Two people taking turns to play your game at your house would be fine as well (given that you don't charge your friend). Two people playing multi-player at the same time on different computers wouldn't be fine with the gaming industry, because they both experience the content at the same time.

Consider that these are old games. 10 years ago, it was not uncommon for games to allow you to spawn multiple LAN multiplayer only copies off of a single bought game. Not only was this legal, it was a conscious design decision.

If it's on a second computer in the house, who cares?
It's no different to allowing someone to play the game on your PC, in this case your second PC.
As long as you don't give it to someone else, everything should be just fine.
Heck, you might even convince someone to buy it because he/she would like to play it at home or in an online game with you.
I don't see the problem, although Blizzard did include special 'Spawn' versions of their games for multiplayer... ;)

I know Tyrian has support for two players on the same PC and it's a free 10mb download, but it's mostly just an arcade shooter.

Heroes of Might and Magic 3 lets you play hot seat (players changing seats to play each of their turns) and I wouldn't be surprised if the older versions did too.

Basically, just read on the game page if it says multiplayer, and in the game description if it specifies what kind of multiplayer it is.

Someone should make a gogmix of co-op games available. Wouldn't be surprised if it's been done already.