How to work with 2 or more AC Voltage sources simultaneously?
Yes, it can work, in theory.  The electrical grid actually does this (though they do it in parallel rather than series).

However, in practice, it can be very difficult because you need to synchronize your two power sources.  If they're out of phase, you'll find that the power output is dramatically lower.  If your sources are 180 degrees out of phase, you'll actually see 80V across your load rather than 100V or 120V.  You actually can make it so the load sees less power than it would have without the second AC source!

Worse, if they're operating at slightly different frequencies (say, 59.98Hz from one and 60Hz from the other), they will continuously drift out of  phase with one another.  Since you're almost virtually guaranteed to have some drift from this, there's no way to just plug your voltage sources together without some feedback.

You would need to be able to control the phase of one of your AC sources and have a circuit that is constantly minding the phase difference and correcting i.  This is what they have at the power stations.  Of course, it may not be affordable to have control over the power in this way.  It all depends on your sources.