I don't know who told you to stop or where they got their information, but it's absurd! If you enjoy it, by all means, continue.
Some tips:
Yes, many women stop producing for a time and your older child may get upset, but will most likely go back to nursing as soon as it's possible.
Every woman's body goes back to producing colostrum at some point and your older child may not like the taste, but again, will most likely enjoy the over-abundance that comes just after birth.
The first trimester can be pretty painful when your breasts are tender. Nurse less if you need to and it will pass.
Get the book "Adventures in Tandem Nursing". It will explain EVERYTHING you could possibly need to know about how it works.
Feel free to PM me if you want info on joining a group such as La Leche League. You can also go to support groups at many hospitals. I go to Community North breastfeeding support groups and you would be welcome there weather or not you gave birth there.
Whatever you do, don't let people tell you that you shouldn't be doing it.
Good Luck!