Multiple subs better than single sub every single day of the week. Factor in you need a device to time align them (miniDSP2x4HD + UMIK).
You need a miniDSP2x4HD if going dual subs, not the app support from Arendal. If you aren't aware of this, I suggest you do more reading before making a very expensive purchase as its best to understand the devices first. You need to time align the subs etc.
I've looked this up now, I believe I was planning to get something similar if I went for XTZ, their room analyzer pro II:
XTZ's Room Analyzer II Pro is the complete package for speaker and room measurements. Read on to see how it performs and compares to the competition.
I'm not sure how it stacks against a miniDSP, but the miniDSP + mic would set me back 360€, while the room analyzer would be 190€, this price difference probably has to do with VAT and import tax, as XTZ is european.
I wasn't entirely sure I would need it, as I have a friend that does audio installation, and I believe he may have similar equipment to do this.
So I didn't include this in the price for the dual setup.