Household circuits come in two flavors, 15 amp and 20 amp. Check the circuit breaker or fuse in your panel to see which one your outlet is wired to.
If the outlet is on a 15 amp circuit, you could only run one, but if you have a 20 amp circuit, you could get away with two, assuming there are no other appliances plugged into the same circuit.
The electrical code says circuits should be loaded to no more than 80% of the breaker's rating. for a 20 amp breaker, that works out to 16 amps. Your two lamps would draw 16.6 amps, so you would be right at the limit.