Ok, I'm splitting one microphone to two cameras. Should I enable the phantom power on both cameras or only on one? I'm just splitting with an XLR Y-cable.

Personally I wouldn't do this.
However at the very least you should test the Y-cable to verify that it is wired correctly and only enable ONE phantom power source.
What mic are you using?
What cameras?

Both cameras are Sony HXR-NX5U's, and the microphone is the Audio Technica AT899.

Is there a better solution to run one mic into two cameras?

You know what I could do, I just noticed that the microphone holder has 2 slots, so I could just use 2 of them...

Well you can use the wired AT899 with an internal AA battery.
There is certainly some advantage to using phantom power generally, but when trying to use a simple Y-cable with phantom I think there's more connectors and conductors that put you at greater risk of giant phantom-power noise spikes if a problem happens. In addition, the non-phantom-camera-input may not like having phantom passed into it.
Personally I'd use an mixer for feeding a single mic to two cameras, but of course I already have a mixer in hand.
You could use an actual mic splitter that's specifically built for the job and that works with phantom power.
These devices also have other controls that may be handy such as a ground lift and attenuators if you're passing the second signal to a different type of recorder than your primary.

Ok, thanks for the idea, I'll look up the mic splitter.

I already have 2 of the microphones, so that might be the easiest way to go.

Even If you use a splitter, turn on just one source or Phantom power, connected though the splitter's 'direct', the other mixer or destination would be fed via the splitter's transformer out. Most 'stomp box size splitters do not have attenuators, but DO have the ground lift, which can save the day.