For some low yielding slow growing plants that wouldn't be a problem.
I agree with the others though. I doubt you have those particular strains, so anything you do to compensate for them sharing a bucket, would be better spent on plants in their own pot. You're essentially giving them both 2.5 gallons of soil, but the reality is, one may become slightly more dominate and hog the soil by growing a little faster first and once it gets a head start, it may deprive the other of some nutrients. What if the dominant one turns out to be male?
If this doesn't happen, you'd still do better by giving them 5 gallons of soil each.