Don't split/join (unless you really know what you're doing), try mashing them with the Disk MaSher
to create single files small enough to transfer. Do all preliminary "Amiga" stuff on the PC under a decent emulator, such as WinUAE.
I've no idea if it will work, but full 880KB disks get reduced to single archive files easily small enough to fit into 720KB. Unmash on the real Amiga.
And, you do know about CrossDOS, don't you?