Unfortunately that is difficult to answer as it varies from room to room. At best you will get more headroom so that the subs can perform with less distortion but at worst you can have difficulties integrating the two subs and if you get it wrong you could end up with the two subs cancelling each other out.
My advice would be to sell your existing VX-10 and buy one bigger and more expensive sub instead of trying to integrate two budget subs.