I also requested a multiple-driver option for Speaker Draft app shortly after purchasing it, however, I found a simple way around the problem, which I believe gives accurate results.
1. Select a preset driver from the list or insert TS values for a new one you intend to use.
2. Estimate the dimensions for the cabinet you want and Calculate volume and reflex port dimensions for just one driver.
3. Double the volume of the enclosure, and this should be suitable for two identical drivers and two identical reflex tubes.
If my design does not work out well or sound as it should I will post again to say so.