How about, 2 versions of buff hammers,
1, For gunners that does the extra 20% damage on weapons, only usable by gunners,
2, works on components only usable by engineers,
Think it might be hard to code in because i think weapons are classed as components but it might solve a lot of issues, if you want the extra damage you sacrifice the repair ability, sounds like a fair trade off, and your 2 engineers can still run with main and buff,