Actually, the cables can be much longer than 5m if you mean HS transmission, see SuperUser Maximum length of a USB Cable.
However, running two USB2 links over two unshielded pairs inside one standard CAT5/6 cable will create substantial cross-talk between two channels, especially over such long cable. If USB devices are HS (480 Mbps), two host ports might see unwanted/unexpected signals, and shut themselves down. More, CAT5/6 cables have 10% higher differential impedance, which increases inter-symbol interference and makes more noise. And lower quality cables (CAT5) make the situation worse. So I am not sure if two USB2 channels over the same UTP networking cable is a good idea.
If anything, you should try Shieleded Twisted pair cable, like this one:
You might have a better chance with that. And obviously, higher electrical quality of CAT6 cable will be quite beneficial for USB HS signals.
Regarding power over USB at 5m distance over UTP, the common cable wires are not that thick, 28AWG, so the voltage might drop substantially. And you will need individual VBUS wires for each USB link, so you can't use a pair of them to increase conductivity.