You should not be able to e-build and provide cover fire simultaneously. 2 or more Riflemen have an advantage over engies in that they can kill enemies while they build. Summary: Remove the ability to fire your weapon while building. This could also go for sabotage and defusal, but those are less of a issue.
Wait, is this "feature" not well known? You don't have to look directly at what you are building to use e-build.
Err... you just have to be near it, I think it works best if part of the building is under you, or touching you...
Not sure why you'd want two riflemen e-building when you could have one engi building with a wall and sentry for support and one Rifleman who can get healed or revived by the engi
I've done this with turrets. I'll lay down behind it and to the side a bit. You can shoot on the side you're on and see the other side while building. Its not much of an advantage though, wouldn't worry about it.
I think the OP is correct if I understand it correctly---that Rifleman are firing weapons while assisting in the building of structures. Shouldn't be able to do both at the same time (assuming that it does happen already)
You said "force build" which I took to mean as in a Jedi mind kind of 'force'. Hence Spaceballs the movie (y'know Use your Schwartz).
Preventing weapon fire while pressing the use key would probably come with a bunch of bugs. Also it is RARELY a good idea to build anything as a roflman. I would much rather be attacking a roflmen trying to construct a barracks than an engineer. If the engineer kills me I fail, and the building is up. If the roflmen kills me, I laugh, respawn and go attack again.