Impedence is dificult to measure w/ a meter. If you want to accomplish (almost) the same thing, measure the DCR of an individual driver and use that spec to determine the desired value of whatever your series/parallel total should be. If the measured value varies much from the predicted value then someting is wrong in your wiring.If you are just trying to get lucky and experiment w/ with different wiring variations w/o taking the time to calculate (or learning how to calculate) a practical wiring scheme then just make sure the DCR of you combo is above that of a sigle driver. This will not necessarily provide good performance but it will keep you from killing your equipment. Sorry if this portion of my answer seems off base but I have no idea how much experience you have w/ this kind of thing and I am trying to keep you out of trouble.
Aaron D