This is fun!
1 target per lane will mean more targets are being shot. But it will increase other factors - most important of these is FAFF.
FAFF - the time spent on a lane that is not included in your 1, 2 , 3 ... minutes.
More lanes will increase FAFF. If FAFF is constant for a lane, because usually FAFF occurs before shooting rather than after which is known as TWADDLE, then having more targets in a lane decreases the proportion of time spent FAFFING rather than shooting. For example - if a shooter uses 30 seconds of FAFF time then once the scope is eyed then the shooter has n minutes to finish. For 1 target this is 1 minute and FAFF represents 50% of this time, for 2 targets this is 2 minutes and FAFF represents 25% of this time. For 3 targets this is 3 minutes and FAFF represents 17%.
So the secret is less FAFF