I cannot see why you cannot plug and clamp the single hose going into the duo valves, it should be easy as you have over a foot of hose free to get at. and do the same to the hose coming from the firewall by the power brake booster also with ample hose to access. All this after lowering the coolant level below the open end of the hoses.

It may be a good time to drain and flush the coolant anyway as the wrong coolant could have been used at one time and caused the problem in the first place.

You could if you felt like it join the two ends of these hoses to each other, with a hose extension using two barbed fittings and four clamps. Plugging both ends would be simpler and act just like the valves were closed.
The duo valve could be opened, cleaned and stored indefinitely.