You don't need to have both as soulmates, each of them should be independant from each other.
However I just checked and the conditions I had for the Intimate actions require you to have them at 100% Soulmate, basically a perfect score on their quests.
I can see this being a bit too strict, so I will have to tweak this around and do another release.
I'll try to tweak this soon and do a release.
Thanks for keeping me updated on this!