Can I dim LED lights when there are two different switches for the same fixture?
No, you should not have to replace both of the existing switches on each circuit. You won't be able to dim from both ends, of course. You just need a "3-Way LED Dimmer" and follow the appropriate wiring. Replace one of the switches (on each set of lights that have two switches) with the dimmer - the other switch will turn them on or off, at wherever the dimmer is set.