I noticed during TI today that the field barriers do stack in dmg when you're in a team at least.
I had Danzo while my partner had Onoki and my ninjas were hitting much harder than I did when I soloed it.
I'm not 100% sure on this but im 99.99% sure the buffs stack only when you're in a team and the other field ninjas on another player's team.

I'm almost certain it does. The dmg output was on a whole other level.
If you can test it out? i'm going to re-test this out as well with a friend.

ah we're both fire mains. possible why then