I have not seen any double tank stand plans, but this is not too different from someone building a large tank with a large sump underneath. Although in your case, that bottom tank is going to be much heavier than any sump.
The main problem I see is how you can support the top tank without a center support. <A center support will make it difficult to service the bottom tank, plus it would spoil the view.>
My current stand is basically a double decker - an 80 on top & a 30 sump in the bottom. But since I don't have to worry about looks at the sump level, I have (2) center supports transfering weight from the top tank to ground. Most plans I've seen call for no more than 24" span for tank rails. To do a 48" span (for an open look for the bottom tank) will need more than double the size of the top rail to prevent sag. I would prob go 4x oversize just for peace of mind. <I am extrapolating from the joist size increase needed to span various length when deck building.> Since the min. to support a big 4' tank is a 2x6 (on edge) with a 24" span. I would say use at least Two 2x8's (glued & screwed together) as your load bearing rails for the 48" span needed. <May be 3 ..... that would be overkill, but better safe than sorry .... Even more overkill would be to use an engineered beam ... either metal or wood ... I have one of those holding up my house addition.>
You would also need to bulk up on the 4 corner legs (since you won't have that center support leg.) 4x4's would prob be enough ... but no harm in going 6x6.
You might see my philosophy as ... overbuild massively so I can sleep at night!