Is it possible to have S1 licensed LFS on two operating systems (linux and windows) on the same computer.
I am asking because i know you can't have it on two computers. If i login on linux for example, restart and login to windows will it cause problems?

You CAN have it installed and unlocked on 2 computers, you just can't use them online at the same time.

You can also run LFS on 2 operating systems/installs provided that you have 2 separate installs of LFS.

Problems can happen if 2 OSs try to use the same *.xyz files, as that's where the unlock info is stored AFAIK.

Yep, correctly stated Degats everything. The best thing you can do, is to have 2 installs. I play on windows instance and do some random visits or just stopping by for some short server invitations on linux. (main OS)