If you have a pay per ride metrocard you can use that on both systems as it deducts money for each fare swiped. However because each system has a different bas fare (path = $1.75 and MTA = $2.00) you cannot use day passes or other multi day discount cards for both systems. One or the other.

http://www.panynj.gov/CommutingTrave...tml/fares.html

3. Q: Will MTA MetroCards continue to be accepted on the PATH system?

A: Yes. The MTA Pay-Per-Ride MetroCard continues to be accepted on the PATH system. MetroCard bonuses or discounts are consistent with New York City Transit fare policy, and subject to change by the MTA. Please note that PATH accepts only Pay-Per-Ride MetroCards. PATH does not accept MTA Unlimited (time-based) Passes, disabled farecards, senior farecards, student farecards, or other types of MetroCard.