Let's see if I can make sense of what you are asking . . .
First, yes, I have used Sunny Islands as you are describing -- the output is tied to the grid. It is rated to be grid tied. It does this by shifting and checking the base 60 hz frequency. If the steady 60 hz of the grid is not present, it disconnects its output from the grid.
Does your other AC inverter source have such a frequency detector? If not, it is probably not suitable to tie its output to the grid, as it could backfeed power onto the grid during grid fault condition. (dangerous for linemen and your neighbors).
You could still use the additional inverter as a back-up power source with the use of a transfer switch, if you would like.