(saw this post even though im not in your area)
I run 2 x 45g sumps on my 300g. Here is how my system is setup:
Overflow from the 300g/46g into the first sump at full speed (into a
filter sock). i have a bulkhead very low in that sump that feeds an external pump for my skimmer/return for small 46g display. Set very high in the sump i have a 3" bulkhead.....The water level is maxed in this sump because of it. The bulkhead connect to hard pvc into my 2nd 45g sump at a downward angle. The sumps sit at exactly the same height. Because the bulkhead is so low in the 2ns sump, i only need to have about 10g of water in there, which incase of a powerout gives me breathing room when it fills to about 4" from the top. The 2nd sump contains about 50lbs of live rock, which is mainly used as a baffle. The an external pump returns the water through the chiller then back to the 300g display
Hope that made sense!