Lol, wouldn't it be best to have 2 Semi-Upgraded Black Dragons, as I think that Fatt_Shade has suggested.
You have 1 Black Dragon, then trough battles, you upgrade it to 10th level (which I believe is the max level), then you go and buy yourself another Black Dragon... Now, in this mod, you'll have 2 x 10th level Black Dragons, which is not fair from some perspective. Instead, as in HoMM: Wake of Gods, you should have 2 x 5th level Black Dragons, which I think would be better.
I think that it wasn't done on purpose, rather, scripting out such a system take's time, knowledge and patience, and modders of Red Sands lacked one of those things, but then again, that's just my opinion.
Anyway, it's a great mod, and we'll have to play it as it is.
P.S. with my relatively humble knowledge of Russian language, I would be willing to help with the translation, if my help is needed.