OK, I'm probably all wet here, let me know where I screw up.
The new plumbing you put in should be the next size up as shown in bold. You will also want the next size up circulator. It can go on either side of the boiler but inside the checks. Wire the coal boiler so when powered it always has a call for heat. Set the aquastat on the coal about 10* higher than the gas burner. When you start your coal boiler, once the water meets the minimum temperature setting, the circulator will start. It will circulate a much larger volume of water than your gas boiler has so you will have a larger amount of heat stored. This mass of water will circulate between the two boilers continuously and keep your gas burner from firing. While unfired, your stack damper prevents heat loss up the chimney. You're domestic water is heated. When your coal fire dies, the temperature drops to the low setting and the circulator stops. The flow/check valves shut and you operate as before on the gas. When closed, the checks prevent you from heating the coal boiler and drafting a heat loss up the chimney.
I recommend that you do install isolation valves too for servicing. They should go close to the TEE on the coal side and the F/C valves inside of them. The coal boiler should have its own air bladder and air scoop
So do you guys think this will work?
Or will the gas boiler lose temperature with long heat calls?
Here is the rub as I see it!
All the energy of the coal boiler must pass into the gas appliance before it exits to the load
all the spent energy of the load also must pass in and out of the gas boiler
a great deal of the load begins to circulate cold water back into the gas applaince and the coal boiler is not heat soaked to give up enough energy ---- there is no way to temper the energy out either to the load or back to the coal boiler and the Gas boiler must fire or both appliances will fall below condensing level. There is no way to balance the energy out for lower circulation temps to level the load on the coal boiler.
Now I don't say my design is perfect but it does elevate this when the gas boiler is off and my pellet boiler is attempting to carry my load alone.
Perfect design is to build a primary loop pumped anytime the load calls for heat. Then heat that primary loop with both boilers (clocked as Greag outlines or with a lead/follow Teckmar control) on separate pumps with supply and returns into this primary loop and supply the load off that primary loop.
Benefit of primary loop pumping - place a small colored light (where YOU can see it) to lamp when the appliance is above circulation temps and pumping energy into the load. There is no more satisfying sight on a cold night - then looking out the window and seeing those pilot lights.