How can you play 2 characters at the same time on EVE Online on 1 account i saw somebody doing it but i cant do it too

You can't.
If you saw someone do this then that is not allowed by CCP and I do not recommend you do it as it can lead to being banned.
Also what you could have seen was someone using 2 accounts to play 2 characters. This is completely supported and even endorsed by CCP.