Yes you can, as long as they do not use common files.
As an example you might have installed mod that represents a whole F1 Season like F1 1998. This will involve new cars skins, drivers and team changes. If you install this mod, you cannot also install another mod that effects those files that have changed, especially the database file (with drivers and teams).
You can install multiple mods such as new helmets (as long as they don't change the same game helmet number), new track signs & graphics, new tyre mods. All of these can be installed at the same time.
Regarding helmet numbers, each helmet has it's own file name, and they are numerically based, like :-
gen_01_high.pssg
gen_02_high.pssg
.......
gen_25_high.pssg
So obviously, if your mod changes the same helmet number as another mod, one of them will be lost.