The situation isn't really that complicated.Two pairs of wire, ran from one boiler to another.
1st. The coal boiler needs to disable the other boiler when it is up to temp. A strap on aquastat on the supply pipe as close as possible to the coal boiler will open on temperature rise ( when the boiler is up to temp ). The 14-2 wire will break the black wire that goes to the burner (oil gun ) leaving the existing circulators , zone valves, controls all still working properly on calls for heat. Also the system will return to its original state if the coal boiler cools down.
2nd. To address the coal boiler lag on a call for heat, a pair of wires(18-2) , runs from the T-T terminals on the coal boiler to a relay that is powered by the 120volts that powers the oilgun (burner). the 18-2 goe across the n.o. contacts of the relay, completing the low voltage circuit to the T-T on the coal boiler when the oil burner controls call for heat. ( which is disabled ifcoal boiler is hot) Or just put a jumper across T-T and let the boiler cycle on high limit.
This setup or variation is reliable, easy to install, easy to understand and still allows flexibility in the hookup. It can be used for any primary/ secondary piping, series piping, parallel piping. Almost all. There are exceptions, but if you can figure those out I wouldn't have anything to type about.