Other receivers with 2 tuners only use them one at a time, so I hope the Azbox makes extra tuners available. Personally though, I think I'd prefer a 2nd DVB-S2 tuner, but an ATSC would be OK too.
As I know, Sonicview Elite 360 has a dual tuner receiver and offers enjoyable Pic-in-PIC two channel watching, or recording one channel while watching another. The receiver uses both tuners simultaneously, not one at a time, because its RAM and processor power are adequate for the task, which is NOT true for older AZBox models when it comes to HD signal processing. They can possibly impose a limitation of using both tuners only in SD mode, or using only one at a time (it would make sense for some people, willing to watch only one channel at a time), but it requires significant extra development work to support ATSC tuner chipsets, and would be contrary to AZBox company fiscal interest to do it now, when they prompt people to buy the Plus. There may be also inherent old MoBo limitations that prevented the company from adapting one of broadly available ATSC NIM tuners long ago. NA is the richest market for them, and Europe is eager for IPTV support right now, so I wish you good lack with this wait. Even if they'll offer an ATSC tuner later for the Plus, I'd expect it to be hardware incompatible with older AZBox models with no explanation why. Too long they enjoyed very high profit margins in NA, but now they may have to compete hard with low cost CN Linux receivers finally ready to restore here fair market rules.
P.S. The DVB-S2 tuner for older AZBox model has being always available (its the same as the tuner supplied with the box) and cost in bulk $6-8, while could possibly be sold for $80-100, if AZBox wanted to sell it separately, offering to use one tuner at a time. Obviously they predicted numerous complains asking to enable dual tuner functionality, and decided not to sell it here. Further, they blocked 2 tuners recognition in firmware in case, one can buy a used second tuner. They will not sell it for older models in the future for the same reason, and on top they want to sell the Plus now.