Some things to think about I suppose are the fact that hot water expands when hot.
So if its overheating for some reason, it will expand back into the cold cistern/tank. Also part of the expanding water goes up the vent pipe, if that is blocked obviously it will go up the cold feed.
If you have a pin hole in either closed circuit coil inside the cylinder and there are pumps on the circuits the same applies really. The force of water from the circuit with the hole in will fill the cylinder and make it expand back into the cold supply cistern/tank.
The same can happen if you have any cross flow connections on the hot system anywhere.
Its just a case of looking to find out what it is.
You can obviously check by feel whether the hot water is unduly hot. Then you can visually inspect the hot system for any cross connections, look for things such as plain mixer taps without nrv's or none bi flows, any pipes cross linked and so on.
The pin hole in the closed circuit coils internal to the cylinder, would probably show up if the hot water in the cylinder is left to go cold and the two systems are also cold, then turn each closed circuit on and off one by one, observing in the cold cistern/tank whether you get any rise in the water level or not
You can also check whether either of the two systems is refilling from their feed and expansion cistern or loosing pressure on the gauge.
Which may indicate that the circuit water is going into the hot water system and back into the cylinder cold feed cistern.
If you can clarify things a bit more we may be able to help.
What are the two hot systems you have?
Are they perhaps solid fuel and a gas or oil boiler?
Is the cylinder a thermal store, with twin coils?
Good fortune
