2 subwoofers - 1 brand?

If your subwoofers are to support a home theater only and your receiver or pre-pro has some form of LFE calibration your main concern could simply be if the second sub you choose has a low level (RCA) input to get signal from your primary subs RCA output.
Beyond that, finding the best locations for two subs to load the room monaurally can be accomplished by employing the crawl test method.  

I am not a fan of stereo low frequency, or subs along side the main speakers locations unless the room dictates those locations. IMEx its more important to have a multiple subwoofers properly located than they be matching models.  

The Piega P Sub-1 MKII appears to have slightly more than fundamental subwoofer adjustments including a phase-switch as well as a phase-shift and a filter-Q. 

If your considering two channel support, in 2008 you questioned about two subwoofers that offered different forms of auto and auto/manual equalization/room correction which may go well beyond an HT receivers LFE. Both had the ability to be used as a master controller for a multi sub system which could be a consideration for a two channel system.

Have fun with your search. 

@swampwalker do you think two rels, one a 328 the other 305, would be close enough. The one main difference is a passive radiator on one and none on the other. Otherwise, both have 10 inch woofers, are sealed, class D amp, high level connectors. Thanks. 

If you add a 2nd sub, I would do it as a stereo sub which means you will need the same brand and model.