Most often the failure mode in power supply electrolytics is such that tacking a good cap across a bad cap can restore performance and this actually has been discussed here a lot. Though you're not likely to find any thread titles that address this for a forum search.
This common failure mode for electrolytics may not always be the case. In the event that too low of a parallel resistance has developed in the failing capacitor then the tacked in 'good' cap would still have this same parallel resistance across it. In many applications, like power supply and decoupling circuits, a parallel resistance defeats circuit function regardless of the capacitance across it. And, you didn't ask but the common failure mode for film caps would be to develop such a parallel resistance, aka: "leaky cap". Same as above for the electrolytics. Paralleling a good cap can't do anything to correct or detect this.