My two boys are 13 months apart (now 6 and 5), I used an Emaljunga buggy suitable from birth (think it was called a husky) with a clip on toddler seat that secured to the front - this meant that it was not as long as a tandem, so not as difficult to manouver up kerbs. it also had suspension and fitted through doors - great for country tracks as well. It didn't give as good a view from the back seat as some, but by the time the youngest was sitting up and taking notice, he was big enough to sit in either seat, so we used to swap them about depending on who was sleepy and wanted to go in the back, or eldest would walk. This might be an option for you especially as no 1 will be walking well by the time no 2 is born and when you only need a buggy for one, just take the seat of the front and away you go.