Thank you all for the input. What I'm trying to do is run a 12v to 120VAC inverter off robot batteries. I want to use two so that you can replace one without losing power.
I think I will go with a pair of diodes. This module with two diodes looks about right for the application:
http://www.st.com/stonline/books/pdf/docs/11644.pdf
(the inverter has two 35A fuses, so I'm assuming it will draw no more than 35A)
Thoughts? Should I give it a larger margin?
EDIT: oops, I missed the .74V forward voltage. I'll find a different one.