Can I use my one S2 account to play with my friend on LAN with my same account?
If so, how?

Multiplayer - start new host - local
For your friend: multiplayer - join specific host - local network,type lan ip adress and here you go!
Have fun dueling!

i don't think you can

It can be done only via LAN.

If you attempt to connect online simultaneously though, like both connecting on a cruise server, it will be detected and something will happen to your account (dunno what :razz

Also it's not possible via hamachi, because for anti-piracy reasons the devs have banned the ip's created by hamachi so don't expect to find each other on the LAN like that.

Quote from truepiece :

something will happen to your account (dunno what :razz

It will just say - username is already online to the one who joins as 2nd.

As far as I remember LFS allows to join 3 people for each account you got.

So, if you have 3 LFS S2 accounts it should be working with 9 people to race in a Lan.