Hi Scott, like I've written many times, each system is different, and that 'pipe-within-a-pipe' heat exchanger is 'different' for sure !!
My loop from my heat exchangers out to the coal boiler circulates constantly like yours, so I'm not sure where your question comes from about summer use of a series 'simple' system. With your system, and mine, the coal heated water is always available to heat domestic water.. In a system with both boilers in the basement, plumbed in series, with a DHW coil in one of the boilers, the water would be kept hot, ready to heat water in the DHW coil. It just may not be circulating all the time.
You must have made up some 'creative' fittings to be able to enclose a sealed 1" tube within a larger tube and circulate separate water systems through each.. I use a 30 plate, water-to-water heat exchanger for my DHW...