Trying to save space on the PCB, can I use a single 25MHz crystal oscillator for both ADIN1300 PHYs? I will split the clock with two resistors.
This is not in our recommendation. The issues that would come would be related to parasitic, loading caps, and EMC issues.
However, pin 6 of the ADIN1300 replicates the clocking provided to the PHY. You can daisy chain this signal to another PHY. That means you have used the clock used by the first PHY that is provided by its crystal oscillator.