you'll be heating the boiler
This part is critical. You must put the stand alone water heater after the boiler so in summer the water out of the live water heater doesn't get cold again passing through the dead boiler.
Now the stand alone water heater has to stay live during the winter. It will use far less energy with the incoming "cold" water preheated by the boiler, but overnight the water may cool off too much in the tank if the standalone heater is dead.
Another option is to install a network of (manually operated) valves so the water goes through the boiler only in winter and the stand alone heater only in summer. Relating the setup to electrical hookups, this is called a parallel connection. When the water has to go through both places, that is called a series connection.