I am pg with my 2nd child due nov and I have also been stressing over which type of buggy to use as like you my ds will be 20 when #2 arrives.
I have looked at tandems, but people have said they are quite heavy to get up kerbs etc with the heavy toddler in the front, plus I think tandems dont look very nice.
We looked at double side by side buggies but I travel on buses quite abit and it wouldnt be possible to take a side by side on the bus.
I looked at the phil and teds, thought it was a really bizzare pram, the first stage bit is ok, but i dont like the idea of baby being shut away in the back bit and also by the time my son was 10 weeks old he wanted to sit up a little in his pram and be able to look around. The older toddler seat bit that fits on the back is strange and i bet it gets a few funny looks!
We looked at the Emmajunga and Bertini, similar types of pram which toddler seats that fix onto the front, they look really nice and practical but I was put off by the fact they are still quite bulky and fold downwards like prams did yrs ago, I was unsure if that would be suitable eg, wether we could get anything else in the car once pram is in the boot, so we decided agaisnt this one.
We thought about buggy boards/seat to go but decided ds is probably a little too young and would probably keep getting off and running away, so we have been given one but probably wont try it with him till next yr sometime.
Finally I found the best option for me is to get a double type buggy but that is the same width+ size roughly as a normal single buggy, then I can use that when im out alone with them both and when were out together, we can have ds in the single buggy and baby in the matrix.
I looked at the marco strollers but have heard not so good reports about their service but have found anougher called cosatto duet lite which is probably the one we will get, but not for a few months yet hth xx