The mentioned fan draws almost 3/4 amp...
Since the Denon USB port is capable of up to 1 amp, only (1) fan should be used...
The other point to double check with Denon is...
Have they designed
separate power/current power supplies for the front and rear USB ports so each have a 1 amp capacity?
If yes, then the fan can operate
simultaneously when connecting an iPod or iPhone.
If no, then an iPod or iPhone should
not be connected if the fan is operating.
Or else U will exceed the current capability of the USB ports.
IMHO..
A better approach is to use an independent power supply for the fan, or a trigger/relay scheme using the USB port and independent power supply.
Just my $0.02..