I am currently tandem feeding my week old newborn and my 20 month old son. My supply did decrease at the beginning of pregnancy but he didn't seem to mind. It did hurt when this happened but it did get better. Just be prepared that your toddler will probably increase how much they have when bub arrives but I have to say its a lovely bonding time for your children. I love watching my toddler stroking and holding his brothers hand while they are feeding...its really beautiful!
I suppose the only negative is the negative comments I have received about ds1 taking bubs nutrients in the womb and now taking all his milk. Don't listen to them and speak to your midwives if you are concerned. I found they were the most supportive..my doctor wasn't.