If you have a medical marijuana card in Pennsylvania, can you travel to other states with it that have also legalized medical marijuana?

You cannot cross state lines with MJ, that makes it federal.

Transporting across state lines STILL violates both Federal law AND the law of every state, including those that allow medical marijuana.
MOST states that allow medical marijuana DO NOT recognize medical marijuana cards issued in other states.

As long as you do not cross state lines WITH your pot.
It is STILL Federally illegal.

Transporting it across state lines is always illegal.

No

All medical marijuana is controlled only by the state which has chosen to allow this. There is no reciprocity.

Which makes perfect sense, because the earlier states to legalize this were extremely stupid in their regulation of such things.