I've read that having multiple parent nodes has been a hot and closed debate. Is this still an issue? I often work with chemical equations where multiple precursor molecules lead to the same product.
Rotate the map so the root is on the right.
I use this technique for exhibition planning.
The precursors are on the left and appear to feed in to the node on the right.
You could also make use of cross links. Those can have directional arrows.
The real problem with feemind and and freeplane is that you can put a node somewhere and make it stay there as you edit further. That being said, we've made use of links and hiding links to build some interesting structures on the freeplane side. I don't know if they cold be replicated with freemind.
Wow. I'd never heard of freeplane before. I'll have to give it a try. I think the rotating idea is visually appealing but for the purposes of actually having an organized hierarchy it might confuse things.