If you didn't mind using the whole barrel for one planter, you could place one half cut-side-down to make a base, and then bolt the other half cut-side-up on top of it.

Yeah, that would work fine, I think.  Depending on how thick the plastic is, you might need to bolt on a few pieces of wood to help it keep its shape.

 i am sure it has been done if it hasnt though i kinda have an idea on how it could be done 

You saw the barrel in half and lait on its side then attach 2x4's so it will be free standing.