you shouldn't really need to get another tank and more heat tape. plug the heat tape into the thermostat before you plug the thermostat in--that way the heat tape heats up at the same time and will be at similar temps.

place the probe on the heat tape itself and cover it with electrical tape. whatever you need the thermostat to be set at for it to be the correct temperature in the cage is where you should set it. it may be higher depending on the substrate and how exactly it is placed under the tank. this may take some adjustment over a 24 hour period while the temps settle, but just keep monitoring the hot side in both tanks and adjust as necessary.