agree with riding the wave that no double buggy may be unrealistic, although of course it depends on you but i had similar gap (17 months) and would not have been without it. it is so easy when you can just put them both in buggy and go, rather than dawdling along at your older dcs pace which makes it take ages to get anywhere
also agree with what people have said not too worry too much, i found the 2 of them easier than anticipated really. obviously it is more work and sometimes both need you at the same time but it is lovely to have them close together and see them develop a close bond. mine are 14 months and 2.7 now (i think.. i lose track
) and get on really well, and think as they get slightly older the age gap should seem less and less? so will be able to do same activities together which must be really nice, and would take alot longer to get to that stage with a wider age gap..
the only thing i think and regret now is that maybe i expected too much of ds1 when ds2 was newborn, i mean just that i look back and think that ds1 was such a baby himself being under 1.5 years but by considering him as the older one maybe i didnt give him as much leeway for his young age, if that makes sense 
expecting another now & the gap will be a bit more than last time 20 months, but still not a huge gap. not too worried about it im sure it will work out somehow. probably the most worrying aspect to me is not having space for ds1 in the double buggy so will have to make him walk full time, although will be turning 3 by that time so hopefully will have to just increase his walking speed so he doesn't take all day, or else just put the baby in a sling.
but generally i think with having ur 2nd, or small gaps or whatever else, we tend to worry about it but when it happens you have no choice but to get on with it so you will adapt and cope, and although can be difficult is definitely worth it.
my 2nd was not planned either btw, but definitely glad we had that age gap now so i think it worked out for the best. but it is a shock at first when you find out you're having another one and your 1st is still only small. just remember though they change a huge amount over the length of the pregnancy, for such a small baby its double their life time so the amount they can do and the level of independence will increase quite a bit so things tend to be easier than you imagine it will be when you find out?