Unfortunately Yamato alone doesn't have the power to fight off their tailed forms, He also isnt strong/fast enough to fend them off while simultaneously trying to figure the sage weapons out.
I highly doubt that Yamato has the power to fight against two 6 tailed jinchuriki. Maybe in a 1 VS 1 he has a chance but for now the Kin and Gin bros take this with low-mid difficulty.
Kin/Gin win easily; they don't even need their cloaked forms though Yamato may be able to supress those anyway. Gold and Silver were trolled by Darui, honestly.