It is possible that mixing cabs would work well but as our Americans friends would say "it's a crap shoot". Two cabs will be louder and likely to produce more low end but they will also produce some odd response to a greater or lesser degree.
Just to muddy the water further, as @itu said, the real measure of loudness is dB and it is the measurement of Sound Pressure Level. The two cabinets mentioned have quoted sensitivities of 101dB and 100dB respectively BUT the crucial thing is that no indication of the methodology. The usual way to measure it is feeding the speaker with 1 Watt and measure at a distance of 1 metre. If we take the 15", it is only available in an 8 ohm version. 100dB sensitivity is at the high end of sensitivity for a 15" speaker. It is possible but unlikely.
The 2x10 can be either 4 or 8 ohms and if they measure the 4 ohm cab, they may have used the "trick" of feeding a 4 ohm cab with a higher voltage*. It means that the cabinet would appear almost 3dB louder.
It's all marketing smoke and mirrors.
*2.8 volts, the voltage used to obtain 1 watt into an 8 ohm load. That gives almost 2 watts or another 3dB.