Two level stand, 180 on top..

[Racersk]

http://www.monsterfishkeepers.com/forums/showthread.php?t=402500

look I stacked 2 100 gallon tanks, I think you could just add some more cross bracing and you'd be fine.

The major difference is you were only stacking a single tank above a single tank. The OP is stacking 2 tanks on top side by side(now) this WILL bow the shelf, because there is a bending point in the middle where the 2 tanks meet. You can not over-brace this type of set up. hence the perfered all metal build.

Also, are your tanks 6'? The OP is adding another 2' in span @ 8' wide.

 

[fish_sauce]

Check out my multi tank stand. It may give you some ideas, although, your situation is a little different. Anyways, hope it helps.
http://www.monsterfishkeepers.com/forums/showthread.php?t=404785

 

[biguebs]

Check out my multi tank stand. It may give you some ideas, although, your situation is a little different. Anyways, hope it helps.
http://www.monsterfishkeepers.com/forums/showthread.php?t=404785

Your thread was one of a few that I had read and studied quite intently while researching this!! Very nice build there