Yes, sir — that sounds familiar. It’s not uncommon for stallions to go through the motions of courtship with other stallions in a same-sex environment. You will see many of the typical courtship displays in such a situation, including brightening, head tucking, quivering and pouch displays.
I believe it is indeed possible to order a single female or male Mustang to replace a lost mate. But before you consider getting any replacements, I would recommend that you take whatever measures are necessary to make sure that your aquarium is healthy and the water quality is optimal after losing two seahorses within a week of one another. I suspect that the two females you lost in a short period of time indicate that some sort of pathogen or parasite may be the culprit, and that their deaths are likely linked.
In other words, I am thinking that whatever caused the skin condition that killed your female Mustang also cause the respiratory distress that resulted in the demise of your female reidi. (Many of the common parasites that afflicts seahorses will attack the skin and/or the gills of the seahorse.)
If that’s the case, and the deaths of your two females are associated, your surviving males could also be at risk from the same disease organisms that claimed your females and it would be very inadvisable to introduce any new seahorses into that situation. Don’t obtain any more seahorses until you are certain the disease outbreak has run its course and the surviving seahorses are healthy and out of danger.
How long ago did you lose your females, Nigel? If it would be helpful, I would be happy to run through my usual recommendations for rehabbing your aquarium after an outbreak of disease.
Best wishes with all your fishes, sir!