Yeah, it seems to me that the crawl test wouldn't work with two subs, unless you intend to colocate them (place them side-by-side or one on top of the other). Even then it's not quite perfect, but probably close enough.
The problem I see is that, with one sub and one listening position, you can swap the two positions, placing the sub where the listener goes and then listening in possible sub locations. That's an analogous relationship, acoustically. But what is the analogy for two subs? You could place both subs in the listening position, and then crawl around scouting out two locations that sound even. But when you place those two subs in those positions and the listener at the listener's position, you've undone the analogy. The analogy would only have worked if both subs went to one location or the other. Not to mention, if the two sub locations are not equidistant from the listening position, you'll have to play with phase so that the two are in phase with each other at the listening position.
Integrating two subs is far more laborious than integrating one is. If it were me, the first thing I would do is learn how to use a measurement system (such as REW over on Home Theater Shack). Then, once I have the two subs, I would identify all the possible sub positions and try the two subs in all possible positions that maintain equal distance from each sub to the listener position. That way you won't have to worry about phase between the subs for each configuration. Then measure each configuration looking for smooth response. Only after exhausting all those options and not finding a suitable configuration would I start looking at combinations where the subs are different distances from the listening position.
For example, my room is represented in the attached picture. My available sub locations are along the front wall and about a third of the way into the room along the side walls. A handful of possible configurations in which the subs are equidistant from the listener are shown, and they are color-coded. I would measure all of those configurations hoping to find a suitable one.