Here is a quick Q&A I did with Anthony Lucas of Eminence speakers.
300guitars: Is it okay to combine two different speakers that differ in sound, wattage rating and efficiency?
Anthony Lucas: We absolutely encourage this! You can get a lot more fullness, detail and complexity if you find two speakers that compliment each other in a manner that appeals to your ears and compliments your gear. Helping you choose two speakers that will do this is a HUGE challenge. It’s difficult enough to suggest one speaker to someone due to the subjectivity of tone! What you think blends well may not be pleasing to someone else. The important technical advice you should know on this subject is to choose two speakers with the same impedance. If you don’t, the speakers will not draw the same amount of power, and also, a mismatch could be harmful to the amp. The other important technical detail to know is the sensitivity or SPL rating. You will want two speakers with SPL ratings within 2dB. This will ensure that one is not significantly louder than the other. It is acceptable to mix speakers from different series and even from different manufacturers. Just understand that comparing SPL from different manufacturers may not be an accurate or apples-to-apples comparison. Not all speaker manufacturers measure exactly the same way.