Materia Meld Question

[REDace0]

I'm guessing either A: They want double melding to require a bit more work, or B: The system they use for such requests can't easily handle the chance for breaking.

Could be because meld chance is affected by melder level. You'd want a L50 when trying to double meld in all cases for maximum chance, but few items actually are level 50, so you'd be potentially reducing your chances having a random L49 ARM try to meld a second Bloodthirst IV on your Cobalt Cuirass as opposed to an L50 you know.