I have two Yamaha synths - a Montage and a Motif. I know I could connect each of them separately via the usb ports on my computer but as I'm running out of these can anyone tell me if I can use a standard usb hub to do the same job?

You can connect the MONTAGE to your computer via USB (using the “Yamaha Steinberg USB Driver” for your computer type and operating system).
Then connect the Motif to the computer through the MONTAGE... The MONTAGE can act as both a MIDI and Audio Interface for the Motif.

Connect the 5-pin MIDI OUT of the Motif to the MIDI IN of the MONTAGE.
Connect the 5-pin MIDI OUT of the MONTAGE to the MIDI IN of the Motif.

When you set the MONTAGE to MIDI I/O = USB, the MONTAGE itself will be referred to as: Yamaha MONTAGE-1 (Port 1)
The Motif will be referred to as: Yamaha MONTAGE-3 (Port3)

Connect the Main L/R audio Outputs of the Motif to the AD IN on the MONTAGE.
The Motif can send audio through the MONTAGE to your DAW...