There's 16months between my no.2 and no.3, so no.2 was about 7 months when I fell pregnant. So I was feeding her about 4-5 times a day until she was a year, then cut down to twice a day like you are doing. Somewhere between then and birth of no.3 I dropped down to once a day (mornings), and continued to feed both until no.3 was 3 months old. Not sure how I managed that now, as dd1 had started school and mornings were busy!
Anyway, clearly didn't affect conceiving. Feeding did get uncomfortable for those evening feeds, and she was messing about, which is why I stopped them. Also, in later months of pg I was clearly producing less volume - she would break off feeding to have a drink of water . Given your child is already 20mths, I think it would be likely that she would stop feeding when you are pregnant, but as this would come naturally it would probably feel ok?
I only rarely fed them at exactly the same time, and didn't like this, just felt odd. Was nice to still be feeding no.2 though, as with the small gap, she just seemed so little to not be my baby, iyswim. I've fed them all to about 20 months, which for me was just the right sort of time to stop. Different for everyone, and best not to worry too far ahead, as they change so quickly at these small ages.
Oh, other thing was that I had difficulties getting started with feeding no.1 and no.2, no problem with no.3, perhaps because I had no break between? Although no.3 definitely opened her mouth wider than the others had, which may have made the difference.