Two 16 ohm cabinets, if you are running them in parallel, equal an 8 ohm load for the amp. So you might be okay, as far as impedance matching is concerned.
It is my understanding that running with too low of a speaker (load) impedance will draw mucho current from the amp, and many amps can't handle the rush.
You may want to test things out before using the two cabinets in a performance, because if the speakers in both cabinets are not an identical match, or if they don't have the same sensitivity ratings, you may end up with more volume coming from one cabinet than with the other. Worse, if the impedance is too low for the amp, it'll blow.
Note: this reply from Buddy was originally posted on 3/16/12.