I wrote similar letters for Australian immigration for 3 separate sets of friends
: (all in genuine relationshps, I hasn't to add!) a while back, and put something like:
"I have known A since [year] and B since [year]. They have been in a relationship since [year], and having known them the whole time I can testify that it is stable, warm and genuine. They have also been living together since [year] in a jointly-rented unit at [address], and both have skills that Australia can benefit from in the long-term."
Of course, only one of those sets of friends is still together and happily married now (still in Australia) too. The others are still in Australia too though, having either married a different Aussie or become one..