As long as you can get output from one sub where you have a null for another, you should be ok.
If one sub has a null at 50Hz but the other doesn't, you should be able to use the time alignment to fill the null with the response of the other sub. This is with the caveat that you might create nulls elsewhere. This is why, in this case, more generally does = better. You have more output so less of chance of all subs having a null at the same frequencies, when distributed around the room.
In REW's room sim, don't be disheartened if you add a second sub an it doesn't do much. Make sure you click the "align subs independently" checkbox, and then feel free to play with each sub's delay to see what difference it can make. I've often improved on room sim's dual sub prediction.
If one sub has a null at 50Hz but the other doesn't, you should be able to use the time alignment to fill the null with the response of the other sub. This is with the caveat that you might create nulls elsewhere. This is why, in this case, more generally does = better. You have more output so less of chance of all subs having a null at the same frequencies, when distributed around the room.
In REW's room sim, don't be disheartened if you add a second sub an it doesn't do much. Make sure you click the "align subs independently" checkbox, and then feel free to play with each sub's delay to see what difference it can make. I've often improved on room sim's dual sub prediction.