Agree that you can't plan the gaps exactly. I have a sister who is 14 mos older than me, and I wanted two back-to-back myself. Then I couldn't get pregnant until DD was 21 months (took about a year), so they are 2.5 years apart. Of course, just my luck, when I finally did get pregnant, DS was born on December 26- pretty much the worst birthday in the world! (I know, I know- first world problems.)
Anyway, ended up being super glad for the longer-than-planned gap. If you only want 2, then maybe you could start trying when the gap would be about 2 years 8 months or whatever you think you can live with, and skip trying the month(s) you don't want to (so when the gap would be 2 y 11 months through 3 y 2 months or whatever it is you want). Gives you a few more "chances" without having another winter baby.
Good luck!