good luck stacking 2 50 gallon barrels when full. It can be done, but you are probably (my opinion) better off just buying a commerically made barrel that is designed to look attractive in the landscape and put it where the neighbors can see it.

having said that if you wish to stack then think brick/cinder block cemented together with the top of one/base for the other using rebar reinforced cement that you pour into a form yourself. Make it at least 3" thick and it should be able to support the wieght of the barrel on top.

the whole thing can then be painted in whatever color works best in your circumstances.

Ours are made with treated 4x4 posts. You should raise them enought to get gravity working for you to get the water out to the garden.