I have two sinewave inverters converting 12VDC to 120VAC (1800W each). I am wiring both of the inverters to a toggle switch to choose the AC power source from either the inverters or from another 120VAC source. Can I tie both inverter outputs (120VAC + 120VAC) together to one switch without blowing something up? (i.e.: hot + hot, neutral + neutral, ground + ground) Or do I need to have a toggle switch for each inverter?

Best to keep both outputs separated. If you connect them together, the sine wave will need to be exactly synchronized to be exactly in phase with each other. This is highly unlikely depending on the components inside each unit.

A real knowledgeable techie with a oscilloscope, and available adjustments inside each unit may be able to get them sync'ed, I just don't think it's worth the trouble.