I am going to build a bearded dragons cage this time, but I can't find measurement to hold 2 adult beardies. Is a 4x2x2 feet be okay or does it need to be bigger?I am going to use Melamine to build it.

First make sure they can get along with each other. If they have never met there is a chance that they will not like the extra company. Knowing that, I would make as much space as possible to reduce any dominance issues. But, if they have been introduced before and you know that they can live peacefully that much space will be fine. But, the more space you can provide them the better. But if that's the biggest you can go and your dragons can live peacefully all should go well.

2 dragons can live in a 60 gallon tank and your dimensions are a little bigger than the standard 60 gallon tank I believe.

As the baby grows a larger area will need to be provided. The smallest enclosure for one or two adult bearded dragons is either a 4-foot x 2 foot tank or a 6-foot x 18-inch tank. A single adult should not be kept in a tank smaller than a standard 55 gallon (48 inch x13 -inch) tank. Anything smaller will restrict activity in a way that is not healthy for the beardie

http://www.dogbreedinfo.com/pets/beardeddragon.htm

It doesn't matter how big you make it, beardies should not be housed together. It causes too much stress as they are territorial and one will try to dominate the other. Just build them each their own cage.

CVT and beardie owner