There are rings that boosts skills, for example a Earth Shaker ring boosts ability by 30%. I assume this is 30% more damage. If I have 2 of these rings, does the boost stack?
It appears to depend on the type of ring. By Earth Shaker I assume you mean this? Assuming that all the bugs have been fixed (which they almost certainly aren't) and rings now function as they are supposed to I think a user from this thread says it best:
"Ability improvement rings work by setting an abilities properties to a 30% higher number all other rings add a bonus to stats. You can see this in the abilities tooltip the damage or duration number there changes on equip. Using 2 ability improvement rings has no use but 2 heal on kill rings for 10% each gives you 20% of your max HP with each kill and 2x20% crit damage gives +40%. This is the same in single and multiplayer."
I managed to find 2 instances here and here where people had actually done methodical tests on stacking rings. In both cases they found that stacking rings had no effect but equally there are plenty of people saying that they're sure stacking does work.
I would say that with the bugs and stacking (maybe) working with some rings and not others the only way to be sure would be to do a test yourself whenever you find yourself wanting to stack rings. Equip them and see if anything changes on the tooltip or your attributes screen, then also maybe also do a damage test with 1 equipped and then with both. My feeling is its probably not worth it. Just use different rings.