Are you growing them outside? How big are they? How old are they?

Marijuana establishes quite the vast root system - It's generally not a good idea to have two plants in the same pot or really close to eachother because it causes root entanglement.

However! There are a few members on this board whom I've learned do this purposefully just so they can later cut out the weaker/less attractive plant out of the two.

Best wishes.