If you already have SRX835s as mains, why go with another manufacturer whose frequency response, phase response, and ballpark output do not automagically match your mains? To me, having more "regular" boxes that can be configured how you see fit is more flexible from a system standpoint.
See the SRX 800 manual page 33 - they give examples of cardioid configurations using the SRX800 series subs. The preset guide also talks about it, and presets exist for cardioid configurations.
You can do a cardioid config with 2 boxes in an end-fire arrangement if you have the real estate. You can do a "pseudo" cardioid with 2 boxes stacked vertically, but it is not as effective as a 3-box stack or horizontal array (think of the ground as a mirror, although it is not perfect).
Keep in mind, whether with another manufacturer's all-in-one cardioid box, or with your own configuration - they need space behind them to work properly. Pushing a sub array with directional control against a wall negates any directional benefits gained in the first place.
