"Another option" could be to utilize the current bandwidth by reducing the [current] SWM channel bandwidth of 100 MHz, down to 70 MHz. This would "simply" let 16 tuners access the same "SWM Band" that the eight are now. Guide Data channel stays the same, the upper limit is raised to 2150 MHz from the ~ 1800 MHz now.
I don't know if this is the way they'll do the SWM16, but it would resolve the dual cable problem.
The 102mhz channel spacing seems to be coming from the design of the technology not a choice DirecTV made in implementing it.
From the DBS delivery pdf
Channel Stacking Switch (CSS ) products support multiple polarity bands down a single cable. Instead of switching the entire polarity band down the cable as in traditional multiswitch products, a CSS IC allows just the customer requested 32MHz channel to be switched down the cable. This allows many tuners to be supported inside the home across the 1.2GHz of total BW available. There is filtering required to ensure that each video channel (or User Band) does not interfere with each other, which bumps the channel spacing to ~102MHz. This allows for 12 unique tuners to be supported down a single cable run.