I have the system you want in my house. It works great! Here's the setup in no particular order.
1. Put a 'T' on the supply and return pipes going up into the house.
2. Install shutoff valves on each side of the 'T'
3. Install a dual junction box with 2 light switch. Each switch will power up and power down the respective boiler
4. Install a 4 gang junction box with 4 light switches. Each boiler uses 2 wires from the T-Stat to the aquastat. Use these switches to switch between your gas and coal boiler
5. Install a 't' coming off of the flue pipe and finish the duct work for each boiler.
That's it. Its simple and works great. I just returned from a vacation down in Florida. Just before I left, I turned off the coal by flipping the shutoff valves and light switches, and did the opposite for the gas boiler side. I set the t-stat at 55 degrees and left the house. Its also fun playing with all these switches and valves. I can pretend I'm in a submarine.