When we moved into the house, I installed a 100g Rubbermaid sump downstairs. I plumbed the 75g into it and ran it with a Mag 18 (same pump I used in the apartment).
The Mag 18 was at its head limit as I was pumping farther than the apartment.
I replaced it with a Dolphin Ampmaster and ran it like that for about a year without issue.
I recently finished setting up the 210g (in same room as 75g) and ran it locally for about 5 weeks to let it cycle.
I then plumbed it into the basement sump and installed a Pan World 250PS.
I had included two bulkheads when I installed the sump as this was the original plan.
After two weeks I then connected the 75g to the Pan World 250PS as well.
Now both tanks are running off a single return pump.
I have a spare Pan World 250PS in case something happens. It is loud and vibrates a lot because the RC seller (hidnReefAquatic) abused it and then sold it in "like new" condition.
When I first hooked the new tank into the basement sump, I ran the pump at about 10% for a day so everything would mix slowly.
Things to think about:
1. Shutoff/control valves - Make sure you can isolate either system from the other if necessary. you also want to be able to adjust flow independently as well as be able to remove the pump from the system without losing too much water. A union plus shutoff on either side of the pump, as well as a manifold to control flow to each tank is a good idea.
2. Backflow/overflow prevention - When the electric goes off, a lot of water may need to drain into the sump. My 100g sump is normally only 50g full. The other 50g is for drain capacity from my two display tanks, frag tank, and refugium. Test this and make sure you have enough capacity. I had to lower my float when I brought ther 210 online.
3. Cooling - The basement sump acts like a natural chiller if it sits on a concrete floor. Adding a second tank means additional heat load. You may need to add a chiller depending on your circumstances.
4. Equipment sizing considerations - I planned all along to have two tanks. My Ca Reactor, Refugium, and skimmer are sized to handle the load. If you are combining two systems you may need to upgrade equipment to accommodate the new water volume.
If you want to come over and see it first hand send me a PM, I live in Gahanna. All of the pictures in the world would not give the proper perspective.
