You will be fine. Your dc1 will change a lot by the time your 2nd one is born, so may not be sleeping through now but the whole situation may be so much different by then as he will be out from the baby stage into a bit more of a toddler. Even a few months is a long time in the life of such a young child so don't worry too much if he's not sleeping now there is still time!
I think with most people, you tend to think how am I going to cope with two??? then once they are born, you just get on with it and you find your own way. Hard at times but you get your own little methods and routines and it does get easier.
I had a similar gap as you, my 1st two are 17 months apart, and ds2 was a suprise so I wasn't really thinking of having that kind of age gap either but it just happened, and overall it was easier than I thought. I decided to do a similar gap again for the next one so can't have been that bad!! so ended up having ds3 when ds2 was 19 months
My top tip would probably be get a decent double buggy to make it easier to go out as much as you want, i wouldnt be tempted to go for slings, buggy boards or any of that personally. Much easier to just strap them both in and go. Getting out the house a lot saved my sanity when i had both of them under 2, i found it helpful to keep busy (moreso to keep my ds1 busy really).
The other thing i feel now about my ds1 and 2 is that when ds2 was born I expected too much from my ds1, he was very little himself but cos of him being the older one I think my expectations were beyond what was right for his age, which probably caused both me and him unnecessary stress so it can help to remind yourself that even though they are the 'older one' and are walking around the place, they are still really a baby! It sounds obvious, but i think it can be an easy mistake to make?
It is a nice age gap, once they get a little bit bigger they can play together really nicely and keep each other busy and hopefully will grow up close. Mine are just turned 4, and 2 and half now (and the little one nearly 1yr) and its lovely to see how the oldest one looks after the 2 yr old. They drag each other around everywhere and really stick together.
And makes it easy cos you can just treat them all the same in terms of bedtimes, bathtimes etc, just do them all together not like having to deal with 2 children very far apart in age with very different needs
Congrats and hopefully it will go better/easier than you think! 
