Dave, in the post Jay mentioned, I thought the original setup was two existing, independant light/sensor combinations, each fed from it's own switch, that were to be connected so that either sensor activated both lights; this gets tricky per my original comments. However, subsequent e-mails clarified the situation: the two lights are currently on one switch and two new sensors are to be added. This is much easier, and you are correct that the sensors, assuming they act as a simple relay closures (i.e. SPST) can just be paralled and wired in series with the original switch.
If you need further clarification, I can fax you a diagram (but I'd need to know whether power for the circuit comes into the switch box or the light fixture box).