I don't know why the pages on this are all so un-clear, but here is the simplest way to state it.
The 3335 does single sided scans only, there is no 2 sided option.
The 3345 does duplex scanning, there is no single sided only option.
The oddest thing is Xerox typical denotes single sided as having an ADF (Automatic Document Feeder) and duplex capable is termed as a DADF (Duplex Automatic Document Feeder.
But with this model they use ADF and RADF (Reversing Automatic Document Feeder)
The tell all, is at the bottom of page 3 here
left of the : is originals, right of the : is output
1:1 is single to single
1:2 is single to duplex
2:1 is duplex to single
2:2 is duplex to duplex
And the 2:X is only on the RADF which is only on the 3335
So the shortest possible answer is to buy the 3345 if possible or you won't get the feature.