It’s going to be tuff to have a fuge and a sump at the same level, and have a fail-safe system. Relying on one pump to feed another tank is tuff because of what was previously noted. What happens if one pump fails, and matching the water going in and out of the return section is going to be tuff.
Have you looked at a hang on back Fuge? That may be an option. I run one on my 30g reef tank because I do not have a sump. Just have a HOB skimmer and Fuge. The only down side is the light a HOB emits but you can find ways to limit that.