I nursed my preemie, pumped for donation AND was pregnant and my supply was fine.
I think it dipped a little at first and the taste changed (don't know for sure) but I was able to do it all til I found out I was pregnant LOL!!
Plenty of moms tandem nurse, and I will too if I ever get the chance. I had to wean my first daughter because she was premature and the Dr didn't think I should nurse since it could cause 'complications' with the pregnancy
My second daughter self weaned at 18months and I got pregnant the next month!

I was still making milk til about month 6 in the new pregnancy.