Can I add two motion lights to an existing circuit?
Lazy Way

Most likely the answer will be yes. Most circuits have at least a little bit of head room on them and if you plan on removing the current receptacle you will be reducing the potential load.

You could also put in some compact florescent bulbs to reduce your overall power usage on that circuit, but I would still recommend making sure you have enough head room on your breaker even if someone puts in more powerful bulbs.

There is a good chance you will never have a problem, but if you want to make sure everything is fine, read on.

Proper Way

You would need to start out by figuring what else is on the breaker. The simplest way to do this is to flip on all of the lights in the house, then turn off the breaker. Walk around to see what turned off and make a note of what all turned off. You will also need to check the outlets. You can buy an outlet tester, or you can just take anything that plugs in, like a radio or a lamp, and walk around to every outlet to see which ones are no longer on. Count the number of outlets that you see.

Basically what you are looking to do is add up the max current that you expect the circuit would ever need. So for lights, look to see what the max wattage bulb you can place in the socket. You can find out the current it pulls by using current = power / voltage. Then for outlets think about what all could be running at once on it, vacuum? tv? etc. Add all of those up, then just make sure you are still under the current rating for your breaker.

This question, Is there an average number of outlets that are wired off of one circuit breaker? , talks a little bit about how many outlets you can reasonably place on a single circuit.