I would definitely go the side by side twin pram. My girls are 12 months apart so I needed "real" pram seats for each of them. I am now on my 5th pram for the two of them so I've seen them all.

It is definitely an either love it or hate it pram but I hate my P&T - both when the toddler seat is on top and on the bottom/back of the pram. DD2 hated it too. I had a one in front of the other (tandem) which DD1 hated - the toddler seat at the front in all of them is quite short and she slouches. Its also a pain when the second child grows up a bit and kicks the seat in front. The side by side is occasionally a pain to get around shopping centres but generally is fine and I've found its by far the easiest way to deal with two small ones. I've got a double valco for walking which is fantastic, but huge and heavy when folded. I have a city mini double that lives in the car for shopping trips and its brilliant - easy to steer, individual hoods, individual reclines (including flat for newborn), REALLY easy to fold/unfold.

Something to think of with the add on bike thing is that while 39 cms doesn't sound like much to add on, that is much more than the difference between a single and side by side double. I also agree with comments that its nice for your toddler to be able to sleep if necessary.