While adding a second subwoofer is very often beneficial, I would be cautious about combining subs that are significantly different in size, design, or manufacturer. Even if they measure similarly in frequency response (and they probably won't, particularly if your aim is to buy a second, smaller subwoofer) they can behave very differently in terms of phase, timing, and transient response.
Those differences can create integration problems that are difficult to fully solve, especially around the crossover region where the subs blend into the main speakers. The result can sometimes sound less coherent, uneven, or slower than a single well-integrated subwoofer.
Using two matching subs generally provides more predictable integration, more consistent tonal balance, and better overall coherence. While advanced DSP can help, it usually cannot completely overcome the fundamental differences between two very different subwoofer designs.
Until recently I have two different, yet well regarded subwoofers. I decided to standardize on one type of subwoofer so I sold them both, bought a matching pair of the same brand and model.
It makes the sub integration much easier, and the lower bass sounds much more coherent and "snappy."