Sorry, if I'm making an already confusing discussion, even more so, but I have a few questions about your design. There are four holes in the top for the (now two) Seaswirls, but the plumbing is coming up from within the tank?
The Supergold Darts only have 12' head before shut-off so I assume you have 1.5" plumbing all the way up, and you aren't going more than 6' up. That 4300 GPH quickly turns into 2200GPH with 6' of head, a ball valve, check valve, and a few elbows. If the two Seaswirls are on a closed loop they will also have more headloss than your proposed tidal system. How many effluent lines will you have on your closed loop? I assume you are dropping down to 1" at the Seaswirls. You may only have 1500GPH on your closed loop by the time you finish up.
You may want to consider a pressure rated sump return pump like a Blueline or Iwaki 100 for the two Seaswirls and use four 1.5" effluent lines (reduced to 1" at the tank) for each of the two Superdart closed loops.
One of the benefits of an OM unit over solenoids (other than cost and noise), is that the OM will allow some passive flow through the closed line(s). This is beneficial considering that these lines will be off for 5 hours and will become anaerobic. Also keep in mind that you may have some air in the lines as they fire back up again. The bubbles aren't a big problem for corals or fish, but they do make a mess of your lighting (salt spray). The Om unit will not allow air into the line when it is cycled "off". The OM has a soft on/off cycle so you don't dislodge biofilms from your plumbing every time it changes path/direction, as you would with an abrupt cycle.
I'm leery of turning pumps on and off on frequent cycles, especially pumps that are not magnetically coupled such as the Sequence line. I find with these pumps, particularly as sump returns, perform and sound differently as the impeller settles in varying positions. If you set it and forget it, these pumps are reliable. Pumps with a soft on/off are another story.
Are you using the check valve as an air intake or to physically stop water flow (back siphon)? Is it a swing, ball or wye style check valve?
I'm not sure if you will achieve your tidal sequence if you are just using one pump to go through the cycle.
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