Harry is 21 months older than Grace and it has been fine. Initially I was worried about having 2 with only a 2 year age gap but we have managed fine. Mind you I am not planning on having another one within 2 years. At this stage I don't want anymore kids but if we did it won't be until Grace is about 3. Having 2 close together may be ok but I don't want to have 3 that close!
SIL's third and fourth babies are 11 months apart, but that was a complete accident. That was hard work at first and physically straining, as she got pregnant only 8 weeks after giving birth so she didn't really have a chance to fully recover from the previous pregnancy. IMO that is too close an age gap, but you can't control mother nature!