I suspect you're in trouble here. The USB ports are there to interface the keyboard to a controlling PC or to a memory device.

Obviously you could physically connect the two keyboards together with a USB A-B cable (same as you use to connect the PC to the keyboard, the "Master" end is the flat connector) but Yamaha would have to put appropriate driver and communication software into each keyboard. If they had done this I think they would have listed this as a feature, so on that basis it isn't there!

I have always looked upon the lack of Midi on modern Yamaha "home" keyboards as a very ill-advised move and a disincentive regarding future purchases.