Galthar, for this to work, you both need the exact same mods. One
way to make this work is for you guys to use different cars. If you
can mod LFS to use one group of cars (cartype) to simulate one car
and mod another group to simulate the other, then you will be able to
play together with 'different' cars. It's kinda hard to explain, but that
way you'll both be able to use each others cars. There is no other way
with MecaniK.
With LFSTweak, you will be able to use, for example, the XR GT for one
car, and the XR GT Turbo for the other. Again, both MUST have the same
settings, so you'll both need to mod BOTH cars at same time for it to
work. This is where presets become usefull. Oh ya, LFSTweak is not done
yet, so you'll have to wait a bit.