M.Kris alone gains faster than Joyeuse, and can gain TP faster than the two of them used together. I wasn't sure either, and I had an M.Kris. before Joytoy, so when I obtained my Joyeuse, I tested this out. Dualwielding the two of them (as you suggest), gains TP lightning fast.
Tbh I sold my M.Kris. though as I'm pretty much merited out for the time being, and I'd like the gil for Skadi Body ingredients whenever the 35 drops.