120gal Opti-white

[untamed]

Try it the other way around first....Use the 1" for the main flow, and the 1.5" for the emergency. You'll need to choke back the flow on the primary Herbie lines anyway...you might only need 1" lines.

If the 1" lines can't carry 100% of the flow, then using them as the emergency lines means there is a small risk of overflow should BOTH the primary 1.5" lines become 100% blocked. (Really, a pretty small risk)

I also have a single Dart as the return pump and two overflow boxes. In my case, both the primary and emergency lines are 1.5"..but the primarys are choked off by gate valves so there is a chance that 1" lines would have worked.