Even V6 two lamdas are preferable IMO.
May be start with single Genboard, use its WB O2 on one bank but make monitoring on other bank using external wide band. And after tuning locate onboard sensor on leanest bank? But make preinstall for second LSU sensor hoping on suitable firmware.
This really depends on your engine and exhaust setup.
If you are running a single TB and common plenum, then if the inlet manifold etc is designed right you should get even air distribution, and hence you need the same amount of fuel in each cylinder. This is the basic assumption you make on a 4 cylinder engine - you don't use 1 sensor per cylinder as you asusme they all have the same air flowing into them. So in this case you only need to correct using a single sensor, though preferably it would be in a place where it can see the gas from all cylinders.
On something like a six the point at which all the exhaust runners come together is usually close enough to the engine that you could fit a single sensor there and get reasonable response and control. On 8's and 12's there may be problems, since many run twin exhaust systems, and you can't put a sensor in a balance pipe as there is no flow (just pressure equalisation) In this case your suggestion of using one side to control and monitoing the other with an external unit would be good. Alternatively a small external board that took in the two Lambda signal voltages and averages them could work.
If the inlet manifold leads to enough of an air imbalance that you need to significanly adjust the fuelling bank to bank, then you are likely to need to alter the ignition timing bank to bank also in order to obtain the best performance.
On engine which effectively run as 2 seperate units (the old Lotus V8 turbo I believe is an example) then you do have a problem. But in this case you would also need to run a second MAP sensor etc so would basically need 2 ECU's, and hence problem solved anyway.
