No concern from price or ease? Weld a bung on each primary and run 6.
IMO... AFR meters are tuning aids, and then something to distract you on the dash. If you want a dual, buy a dual. I find 1 to plenty, especially considering you can swap it to the other bank if necessary.
The 1GR's upstream sensors are essentially widebands. You can get a lot of data off of them.
Personally I think double will confuse you when tuning. If you want to tune, just put one. A healthy engine should return almost identical reading on both banks. If you want to check the health of your engine, get an OBDII scanner and monitor Bank 1 and Bank 2 short term fuel trims. If they are close, then you're in good shape.