We have done just that, conceived when DD was 5 months old so gap will be 14 months. I felt ready, and tbh we both wanted another baby the moment DD was born!
I'm 3 months pregnant now, and there are two things I wished I'd thought about before our night of passion:
1: pregnancy can affect your milk supply so think how you would feel if you had to wean onto formula earlier than you planned. It doesn't affect everyone but still. It also makes your nipples sore (again different for everyone).
2: check your contract with work. I have to return to work for just 1 month to get another years maternity. I'm civil service so probably the best terms. I reckon other organisations might need 3 months back at work before.
3: I had conveniently forgotten just how tiring first trimester is. Seriously! Maybe it's easier to do this whilst still on mat leave. I'm not sure, but anyway, my brain/hormones had erased the details of ms etc
Absolutely not trying to put you off doing it - I'm massively excited, and just had 12 week scan. I think it makes sense for my career too - I would only have gone back to work to try for a second, so why not get it out of the way and when I do go back, I'll know it's for good.
Good luck pregnancy hunting!
Also - lovely to hear stories of close babies getting on well as they grow up. Thanks xxx