Wow, that's the cheapest I've seen these. A few thoughts in addition to the previous post:
1. The listings for these inverters caution against hooking to a battery. From that I conclude that their design relies on being able to pull down the input power easily.
2. If the GT inverter is designed correctly there's no hope for it syncing to a non-inverter generator.
3. There's always the risk of frying the other inverter. If the other inverter is parallel capable, IMO it should tolerate this ok. I haven't heard of anybody (including myself) willing to test the theory. This exact scheme is starting to be used with residential PV systems. Outback and SunnyBoy say it's ok with their off-grid inverters.
4. If designed correctly the GT inverter won't kick in until the other power source has been stable for a few minutes.
I've been toying with trying one of these inverters at home. I have to assume they're junk, but the $ risk is pretty low here.
Edit: Did you notice that when you select an actual model the price goes up "kind of" significantly (about 10x for most models)?
