Joining New and Existing Tank to New larger sump?

I'll represent a dissenting voice in the two tank/one sump thing.

I believe that running two tanks off a single sump is an excellent way to go. First off, you get a larger combined water volume for better stability. Second, you only have to buy one set of sump equipment. Some of that equipment is the same as for a single tank (dosing, ATO, etc) while other requires larger (return pump, heating/chilling, skimmer). Though typically equipment for two connected tanks is not twice as expensive as two sets of smaller equipment.

As to setting up a new/additional/second tank, it is not too difficult to add one in. Set up the new tank, plumbing, etc. and keep the new tank isolated with nothing in it. Do a partial water change by swapping water between the two tanks. Fairly easy by simply opening the valve to the new tank for a short period of time. Repeat this process every day or two until the water has been sufficiently mixed then open the valve to the new tank for good.

At that point you can start adding cured live rock followed by livestock.

 

It's no different than doing a big water change. I let the water run through the new section for a couple of days with and auxiliary pump to make sure there are no leaks, then match salinity and temp, and open the valves.

I currently have a two tank FOWLR system with one common sump, and a two tank reef system with one common sump. I would not have enough time in the day to maintain four separate systems, let alone the added expense.