I have opposing OSC cycles for 2 power heads in my tank but I've noticed they are out of sync.

I have seen cases where opposing OSC commands end up being in sync which is why I prefer to replace code like this :

OSC 015/015 ON/OFF Then PML ON
OSC 015/015 ON/OFF Then PMU OFF

with this:

OSC 015/015 ON/OFF Then PML ON
If Timer PML = On Then PMU OFF
If Timer PML = Off Then PMU On