Ah thats unfortunate haha Running just one sump is less hassle the only reason I mentioned it.
Way oversize the bulkhead for the water draining from the rubbermaid to the 20g sump. Thats if you don't feed to rubbermaid off a manifold from the return pump, have a dedicated feed pump to the rubbermaid or T off the DT drain (have a ball valve on the rubbermaid side) to feed both the 20g and rubbermaid at different rates. If you do either of the 3 I listed in my second sentence you could just run a 1" bulkhead with no problems. If you do it linearly so that the DT drains into the rubbermaid and then the rubbermaid drains into the 20g sump before it gets returned. You'll have to way oversize the bulkhead.
Before I just ran 1 55g sump under my 125g DT I ran a 20g long draining into a 30g long tank for my sumps draining linearly like I described above. I found out the hard way by just nearly avoiding a flood, constantly battling salinity swings and having to turn my DC return pump way down. The bulkhead on the first tank or rubbermaid in your case needs to be way larger then the bulkheads on the DT (again if you don't use a pump to feed it or T off the DT drain). I ended up having to run a 2.5" bulkhead in order to get the turnover I needed through the sumps.
I highly recommend feeding the rubbermaid off either a pump (manifold off return or dedicated doesn't matter) or T fitting off the drain from the DT. You'll avoid a lot of problems, hassle and save the money on having to get a huge bulkhead, and hole cutting bit.