Pipe design ideas for vs pumps & 2 boilers

Hydraulic separator

Have you considered using a "low loss header" or otherwise known as a hydraulic separator. They are inexpensive and readily available from Caleffi and Bell & Gossett. Also the Triangle Tube Solos now come with the ablity to run a 10V dc signal to the MCBA which would allow you to maintain a priority for the domestic production and stage the boilers with outdoor reset. Depending on how much water you want to get there are a couple of different piping options to take full advantage of the boiler HP. Piping diagrams are available from Triangle Tube that show this style of system.