Do repair bays stack ? And by how much ?

Even if the repair bays don't "stack" as such there is still the benefit of having the second bay to repair the first while it is being beaten down - and of course once the first one is gone the second one is still around.

That said... my impression is that the repair bays work by charging up their antimatter then spending some of it to repair nearby units. I've not paid attention to whether that's an area-effect ability or whether they are picking specific targets for each repair.

Either way, my systems definitely hold much better with two overlapping repair bays protecting the strategic installations like planetary shields plus the fighter outposts.

They're not so useful with guns because the guns usually have to spread out to protect a planet, but an asteroid is small enough that you can put a huge lump of guns on it and that can work better than fighters. The difference is mainly because the asteroid's low hitpoints mean it is vulnerable to a lone capital ship's bombardment and capships often have abilities that can cripple a fighter swarm.

Repair bays cast single-target heal over time abilities on a cooldown. They don't stack, but multiple repair bays can get more targets with the buff faster, or if one runs out of anti-matter the other can continue repairing.

Thanks for the information and tips!

Will really help in my future games.