Transfer switch is intended to prevent your electrocuting a lineman sent out to fix a downed line because your generator makes that downed line hot, even though the power company shut it off. It is not intended or designed to auto switch between two hot leads based upon which source you prefer to use. If you had a manual transfer switch, you could do that, but auto transfer switch, nope.
As pointed out already, if you ever wind up with both generators hooked to the same power cord, or either generator hooked to the mains, you will wind up with a burnt up generator. You have to do whatever it takes to prevent that.
