When I was researching how to bring back my fertility to try for DS2 while I was breastfeeding DS1 I came across people using it while breastfeeding to kick start ovulation.
I'm tempted by it too because I'd love twins, I think breastfeeding itself increases our chances of twins though. There's a study out there somewhere that says women who conceive while breastfeeding have something like a 12% chance of twins! It sounds too high to me but I hope it's true!