That's a bummer. But why do you need check valves?
I plan to run two Vectra M1 pumps as my returns, each plumbed to one side of the tank. I'm worried about a failure mode where one of the returns fails. Depending on the exact length of the run to the basement, a failure of one pump will create a siphon at 1,800 - 2,300 GPH while the single remaining pump can only pump about 1,200 - 800 GPH; as the head height increases siphon drain increases and pump throughput decreases.
My design for the plumbing has the two return lines connected to each other and isolated by ball valves, which can be switched to make just one pump or the other drive both return lines. This setup is also going to be plumbed into the drain and isolated from it by another valve. A photo will be worth a thousand words here and I'll post one when I've dry fit the plumbing...
In any event the idea of the check valves is that if one pump fails (and the check valve is working) then the other side will be able to keep the tank running until I get home and see the problem, at which point I can switch up the ball valves to drive both returns from one pump (at admittedly half the flow through my sump). A failure that siphons the water into the sump system would require both a pump and its corresponding check valve to fail, which isn't that likely if I inspect the check valves periodically.
What's the glass thickness on your tank?
Thanks
The tank is 3/4" glass eurobraced.