Not a good idea. First, If one is a male and you kill it, its roots will rot and this is bad for the living plant. It invites mold and diseases. Second, if they are both females, likely they will be genetically different and grow at different rates. One will eventually crowd out the other. And if you decide to do something about it, like transplant one, it means major damage to both root systems.

Now, two clones of similar vigor and size, will work as long as you keep them together. Separating them will damage the root system. They should grow at the same pace and if you grow in a big (6 gallons or more) pot or tub, they should have enough room.

But on the whole, it's a bad idea. I start my plants in individual 16 oz cups, then to one gallon pots for vegging, then in to 19 gallon tubs. I've had 2 plants in a tub, but it just didn't give good results unless the plants were clones. - Granny