Actually, I'm thinking of soing exactly the same.
For me, both Eheim 1250 pumps are submerged into a res. Radiator used is a Silverprop4 with 4 120mm Sunons at 7v
2 loops are used .. just like yours .. One for the RAD the other for the blocks.
Both loops end by dumping water back into the res.
Problem is....the water in the blocks loop may actually circulate within itself, thus making it higher in temp as compared to the Radiator loop. Also, having 2 pumps subermeged may also casue the water temp to rise!!
To counter this, I've decided to place the outlet valve of the waterblocks loop near to the inlet valve of the radiator loop (which is the pump). The same goes for the radiator loop. This way, the warm water from the blocks will be "channelled" into the radiator loop and the cool water coming out from the radiator loops will be channelled into the waterblocks loop.
Although the dual pump dual loop setup will seem to increase flow rates, it might not actually help very much in the overall performace of the setup. Still, it will look very cool and I'm willing to have a go at it
Good luck with yours. Post your results k