My first two are 21 months apart and I didn't have a double pram. I either wore the baby and the toddler sat in the pram, the toddler walked, or, if I was desperate, the baby lay at the back of the pram, and the toddler sat right at the very very front holding onto that bar thing that goes across the front. Lol, it worked really well!
Then when my twinnies were born, my DS was 2.5, and he either rides his scooter (the micro one Fearless leader was talking about), walks or sits on the front of the mountain buggy. You know how MB have that triangle thingy area thing at the front? He just sits on there if he gets tired. Or I wear a twin and he sits in the pram.