When fitting multiple speakers into a single box (such as a 2×10 or 4×12), it is important to ensure that all of them have the same impedance (measured in ohms). If you fit two speakers of differing impedances*, the power will not be distributed in a balanced way which can lead to irreparable damage.
*Note that you can consider 15 ohm and 16 ohm speakers to be of the same impedance.
Advanced information
For best results, the overall impedance of your speaker cabinet should match the output impedance of your amplifier. If the cabinet impedance is higher, then the overall output level will be reduced. If the cabinet impedance is lower then you run the risk of the cabinet being over-powered and damage may occur.
