Wiring 800kHz signal together with RS485 on Cat5
You should also use a LVDS or RS485 driver for the 800kHz signal, too.

The reason is not the other RS485 transmission line, because this symmetrical signals can't produce interferences (almost).
You should be afraid of signals from outside the cable, which can easily disturb your 800kHz transmission. The connected LEDs will flicker in case of transmission errors, this should be avoided.