I have a 125G... on a simple steel stand...
For this I would definetly go for steel......!!!!!! Some nice square tube would do the trick....!!!
I would imagine you could go to any steel fabrication shop and tell them what you want, how much weight it's going to be carrying and they would be happy to design it... and build it.... I would imagine that they have a lot of steel they are sitting on and probably just now starting to get busy....
If you are going to be putting 250 gallons of water = 2,075 lbs
Plus the weight of 2 x 125G tanks 2 x 177lbs = 355lbs
Rocks, filters, substrate, maybe a sump, extra stuff say 500lbs
Now you are conservatively at around 3000 lbs BTW - Thats WITHOUT the stand....!
IMHO.... make damn sure the floor can take that weight!!! A normal floor is only deigned to handle 45lbs per sqft average weight in a typical room... this would be about 375 lbs per sqft.... your house will never be the same...!!!
If this sounds like th voice of experience... you're right..!!!!! It is what a structural engineer told me for $500.00... cheap compared to the potenial probelms.... I avoided... hehe!
Now, if you are going on concrete in the basement then no worries...!!!!!!
Good Luck....!!!
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PS/ If I was going to do all of that, (unless i was keeping intentionaly keeping the fish seperate), I would put a 210 or something like that.... one tank.... or at least get a 150G instead same W& L just higher 29"... you could pump up to the top tank and have it drain down into the lower and then into a sump.... very cool concept... keep us posted....