Sump/Refuge thoughts for my tank

I'm going to be building a sump and refuge for my 55 gal tank but still haven't decided whether I should use two tanks or one. I can't find a used 30 gal tank on craigslist or ebay close to where I live but a petstore is having a special for a 20L tank at $25 and a 10 at ~$20. So my questions are:

1. Would it be better to use 1 or 2 tanks (perhaps make one 20L sump and one 10 or 20L refuge)

a. Is there much benefit to this other than more water volume

b. If 2 tanks, what do I do with the space in the sump? do I also have sand in the sump (basically two refuges)

c. If I started out with just one 20L tank as a sump with refuge and decided later to add a second tank, would the design not be as good if I just initially started with 2 tanks?

I'm adding the sump so I can add more volume to the tank, help with nitrates and other chemicals, move my skimmer underneath the tank, and add a top off unit. my 55g is 48" long so I'd have enough room but I may want to eventually buy a phosban reactor.

I currently have a clown, regal tang, colt coral, some small zoas, and a purple tip anomene. Equipment are T5HO, bakpak skimmer, 2 koralia #3s, and crushed coral that I am changing to caribsea aragamax suger-sized sand this week.

Sorry for long post.

 

There are usually 100 ways to skin a cat, I have always liked the simplest. Don't make things too complicated.

I was going to add a separate fuge to my sump, but it seemed like more possible problems for no benefits. It would not be the end of the world. Plenty of people do, but if you have a choice, go for the easier one.

With the space you fill it with equipment. No sand. A sump is basically just a bucket to put all your gear in instead of your tank. If there is room, some people put LR in it to add to the filter, but no sand.