Should i mount them close together or far apart?
It doesn’t matter, as long as you are not stick them together. But you should mount them as far as possible from autopilot and any other high power devices.
Should i use blended mode or use auto switch 1 (use the best GPS)?
Blended mode gives worse result if 2 GPS are 1 good and 1 bad, because it blends them.
Auto mode depends on the GPS’s self-reporting status. It is better in most of them time as long as the GPS correctly report its health status.
Should i use CAN 1 and CAN 2, or CAN 1 and GPS1, or a CAN splitter on CAN 1 only?
As far as I know ardupilot doesn’t support dual CAN GPS yet, so you need 1 GPS in CAN mode and another in I2C mode.
Can they both be powered by the Cube or do i need a separate 5v power supply?
It depends on how many peripherals you have connected. You can check the maximum current draw allowed from Cube from docs.cubepilot.