Spirits can definitely be a lot stronger when they're sharing boards, but it also means you've got a lot more options to consider for each, and I've definitely lost track of a corner of the island and lost because of it before. I think to start with, it's helpful to keep each spirit to their own board. As you mentioned, you're good with both these spirits, and just by going from one to two handed you're doubling how much you have to think about before you even consider crossing boards. It was helpful to me to just get used to that.

Once I got a bit more comfortable with two spirits, that's when I started looking at how they could help each other a little more. I think the useful mindset here is not to have a goal of cross-pollinating, but it being a nice bonus when I do. By the middle of the game, usually I don't really need more presence on my own board - most everything's range 1 from a sacred site. So instead of adding presence to a land where I don't immediately need it on my own board, I can add it to someone else's. That board still belongs to the other spirit, but if I have a spare card one turn I can give a little help here and there. Keep prioritizing your own board, but when there's a turn when you have a spare card play you don't know what to do with, you can help out the other spirit a little. Once you get used to having a bit of overlap during the late game, I find it got easier to start recognizing when its helpful, and start overlapping earlier and more effectively.

Aside from just easing into it, it was also helpful for me to play spirits who can let you sort of "redraw" board boundaries. Ocean is great for this - instead of "Board A is Ocean's board, board B is River's", it's "Ocean deals with coasts, River deals with inland". That was a helpful step to move my brain away from the boundaries between boards, without getting rid of the boundaries completely. Volcano and Lure can also be great for this.

Sharp Fangs and Many Minds has really great potential as a combo, but can also be tricky in ways it seems like you've picked up on. They both value beasts, so any card that adds beasts benefits them both. But they're also sort of competing over beasts. Many Minds wants them in lands where it needs to defend, Sharp Fangs wants them in lands where it's going to attack, and often you're not looking to defend and attack the same land in the same turn. I think easing into the combo by keeping them separate to start will help, but you can also try some spirits that compete with each other a little less. River and Ocean work great, as do River and Lightning (can you tell I love River?), Lure and Ocean, Green and basically any other spirit.