My solution was to install a single heat valve, driven from one heat shroud. The second shroud is removed from the muffler. The installed valve supplies heat to the rear outlet, which is actually under the front seats, and gives us plenty of heat at altitude with surface temperatures down to about 30 F. Below that, you will need to bundle up a bit, but no big deal, and I don't fly that much when the temp. is a lot below freezing. I have a layer of fiberfax as a thermal break between the valve and firewall, and no issues with a hot tunnel.
The traditional approach to cabin heat just doesn't sit well with me.....we rob cooling air from the engine, and most of the time just throw it overboard, creating excess drag. In the RV-10 with dual heat valves, the flow conflicts and waste heat gets bounced right onto the fuel pump on it way out of the cowl. There are so many things wrong here. With one valve, you are wasting some cooling air in summer, but less, and with one valve you aren't throwing heat onto the fuel pump. Less scat hose back there helps with access, and might help in accelerating the flow out of the cowl too. The more I think about it, I may take off the one heat muff I have, and just dress warm!