hi all,
is there a game that can be played by 2 persons 2 pc (LAN) working together at completing a mission not facing another

I would recommend a classic.

Total Annihilation. If you can get the original game from Cavedog, then add a mod called "Units Compilation Pack" or "UCP". Affectionately known as "TA:UCP". It is a good mod, there are many others but this is popular.

If you try, because it is an old game and Cavedog is no more, you may be able to find a couple copies for free.
It was the LAN game of choice for years over here. Very fun. Basically you and your friend would play as a team against the AI, and you can have more then one AI team, who even fight each other as well as you.

Check it out.

Serious Sam First & Second Encounters let you play cooperatively.

And most real-time strategy games let you gang up on computers, though they're not really missions so much as skirmishes. :bounce:

No one lives forever II.

thanks guys i will check them

The original doom games! They are 2+ players!