I have a six bucket system....and I do have experience with two plants in one bucket...awhile back I had an extra clone so I put it in my clone bucket with another clone...and they grew together...I could not get them apart so I put them in one of my five gallon buckets and it turned out just fine...ended up being a pretty bushy plant....I have not done that again....but it did work....
I run six buckets with 4 1000w HPS....The state I live in I can only have 6 mature plants....other wise I would try It again....heres a pic