After 5 unsuccessful attempts, I was finally able to meld 2 bloodthirst materia IV's into a Cobalt Cuirass red! After the first piece was added, the second piece's probability of melding was ~17% depending on the amount of HP the materia had. Higher HP materia decreased the probability by 0.5-1%. It also helped slightly to affix the materia that gave the lower HP first. I hope this helps anyone who wants to try this in the future. Good luck.

P.S.

I checked on the probability of doing a third materia and it was at 7%