There is no reason you require a second pump simply to run a parallel loop for a radiator. All you need is an ample pump and a throttling valve in the "bypass" line. The throttling valve works a little nicer than separate pumps as you can then split the flow between the radiator loop and bypass in whatever ratio you wish. The downside is that you burn some energy in the pressure drop going over a valve, so overall efficiency takes a minor hit.
Ben is right about sensing flow, though it's not quite so simple as he states. To do it "nicely", you need a couple of relays and a timer circuit. I had an EE at work design a circuit that uses a couple of relays, a pressure switch, and a timer circuit. 12V from the PSU powers on the pump via a relay and starts the timer circuit. If the pressure switch doesn't pick up prior to the timer expiring, the second relay cuts power to the PSU. Sort of brutish, but very effective. Get a normally open pressure switch and relays and it's reasonably fail-safe. Total parts cost is on the order of $30. Without a timer circuit, you need something to keep the circuit closed until the sensor picks up flow/pressure. A spring return push button (just like an ATX power button) can do the trick.
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