USB works like a client / server sort of thing. With a phone or a tablet connecting to a PC, the PC is a host and the phone etc is a slave device.
Connecting a phone directly to a tablet, you’ve got two slaves, nothing’s in control, so it’s never going to work. I think there are cables that have a slave connection at either end and a box of electrickery ing the middle which acts as the host (so you’ve then got slave > host > slave), but I’ve never used one and whether the iPhone will play nicely with such things I’ve no idea. Direct file access to iDevices can be awkward.
I think if it were me, I’d be trying to do if over wifi rather than cables. Or, actually, bluetooth push maybe. Is there any sort of OBEX app for iPhone, I wonder? I might have a play this afternoon, see if I can work it out.
How on earth are you connecting an iPhone to a tablet in the first place?