Never wanted to do that myself, but there's a few things you could try:
- Use the 'reset outposts' option
- Join your partners game. Outposts you liberate whilst playing as P2 in co-op don't save in your own game. So with the same co-op partner you could liberate the same outpost twice, once in each players game.
- Allow the outpost to be re captured.
I haven't attempted any of these myself though.