Other than having more than one in case of attack, do the bonuses stack for multiple starbases in one sector? For example, if I have two starbases, and they both have a +10% military/social, does my planet in that system get +20%? What about the +1 speed for ships?
yes, all bonuses stack, except for bonuses that only affect that Starbases sefense bonus (those modules of the Defense variety). Keep in mind that there are upper-level caps for military and social productionj for each planet class (these are VERY high, however). If you really want to supercharge your production, build up to three fully loaded production starbases in a sector.