I looked into harvest mice once and found lots of good information here:
https://www.rodentzone.com/harvest-mice-as-pets/
I don't know of any forums for harvest mice, a better place for information about exotic species might be one of the Facebook groups like Exotic Rodent Breeders UK or Keeping and breeding harvest mice.
This website looks good too:
Harvest Mice
I've mostly heard of people keeping them in breeding colonies so not sure if they can be kept in same sex pairs. I think mice in general do best in groups rather than pairs, the more the better if there's enough space.