Does combining ac power sources double the voltage?

This is not a Yes/No answer, as the answer depends on the AC power sources used. There are two factors:

- Frequency: e.g. in the USA the powergrid its AC frequency is 60Hz

- Phase: an AC power source its voltage rises up and down with a certain frequency. When you take 2 AC sources, Phase is the shift between those two (this is often expressed in an angle).

If the AC power sources have the same frequency and there is no phase shift, then the AC Voltage will double when you serialize the two.

However, If the two sources are 180 degrees (or PI radians) out of phase, then they will cancel each-other out. Imagine AC current like the ripples in a pond when you throw 2 stones in the water 2 feet apart: The ripples flow out in a circular pattern from the center of the spot where a stone hit the water. Where the 2 circles meet, you'll notice that a some spots there is no activity (the ripples cancel each other out) and at other spots the ripples are higher.