[TerraMex]

Still , my questions presist. You cannot make sure.

Are you also assuming the higher flow loop will interfere with the lower , "hogging" most of the water ? That raises another problem. Water has a tendancy of going through the less restricted loop. The radiator loop pump will interfere with the block loop pump. Some water will be recycled through the same loop. Not necessarely a bad thing in the radiator loop.

The switched inlet - outlet will take care of most problems, but wont eliminate all of them.

may i suggest a simple design?
Take that box that #rotor did, make the inlets and outlets as you stated, but insert a wall between the pumps, with perforated holes ( or similar ). There will be communication between the two chambers , but it will be reduced due to the added restriction. Most of the water from the block loop will flow through the radiator loop and very little (if any) will be recycled throught the blocks loop.