I mostly read and learn, but have recently done exactly this.
Had a 90 gallon main tank and wanted an additional frag tank.
Mine is set up this way.
Main tank has two pumps, one to a manifold, the other directly from sump to tank.
Frag tank is fed by two sources. One from the manifold and a separate pump in the sump dedicated to the frag tank.
The frag tank is a 40 gallon 48x48 and has a twenty gallon sump. The sump has a stand pipe that keeps the water level exactly at 9 inches.
The sump from the frag tank drains into the main tank sump, dumping into, filter sock.
This way, each tank has redundant pumps, one fails and the other maintains circulation.
If the main pump fails, no new water enters the frag system and it becomes closed. The main system also functions because the second pump maintains circulation. If either other secondary pumps fail, the manifold keeps the flow going and both tanks remain connected.
Each sump has a heater. The skimmer is in the frag tank sump because there is lots of room. The main tank has a uv, gfo, carbon and bio pellet reactor fed by the manifold. The main sump houses the main fuge with lots of macro.
Only thing you have to manage is getting the secondary sump high enough to feed to the main sump.
Of course, I probably could have not used the secondary sump, but it give more fuge area and extra room for live sand, macro and skimmer.
Other advantages, only one set of water tests every few days vice two, one ATO, more water for consistency and others.
