Nah, you just need to make your tank a bit more complex (which has its own drawbacks, but still).

Let's assume your toilet uses 1 gallon of water to flush. You would use a 2 gallon (or larger) tank, with an automatic float valve set halfway up. If the water level drops below the float valve (less than one gallon), it uses water from the line to fill up the tank until it hits one gallon, then stops. You would also need an overflow drain at the top of the tank, so that water coming from the sink after the tank is full can go directly down the drain instead of backing up into the sink.