You should be fine. You can nurse both babies no problem. The first one will likely want to nurse more after the next one come too (as they will see the new one nursing).
When my child was younger, he used to nurse for a long time (sometimes up to 40 min). Then he gradually nursed less- like 20 min and then eventually 10 min or less (but sometimes more frequently). Anyway, the point is they do adjust (especially when you are introducing solids). If you are worried about it don't go crazy with the solids.
The important thing is that your child is still getting breast milk which is very healthy (and so much less expensive than formula).
Another thing is that I really didn't give my child much juice or any thing because they get the water/ liquids from the breast milk- so just be aware of that too. They will still come to you for the liquids.
It is a bit scary when things change.
Also please know that as they get older they get better at "suckling" and may be able to get more with less time/ effort too.
Plus your child will go through phases when she will nurse more (if she is not getting enough milk especially) and this will help build up your milk supply (the child can help you make more milk if she is not getting enough by doing a day or two or more of more frequent nursing- just fyi).
As far as nursing two you are fine. Just make sure you are eating healthy. Also make sure you find a Dr that gives helpful advice and doesn't just tell you to stop nursing.
One of my friends was told to stop nursing when she was pregnant, but this is not necessarily correct. When I went to the Dr's she was very happy that I was still nursing while pregnant. Please keep in mind this is not an uncommon thing (ok it is less common just because not as many women are nursing as long these days).
I would encourage you to make some friends or get support to help guide you to (at least so you can ask questions and get more information). Like LLLI (La Leche League). Even if you don't agree with every thing they say, they can be a good resource for you. They have helped me immensely. Also attachment parenting has helped me (not sure what your parenting beliefs are- but they are a more nurturing kind of group. And I do not believe in everything the group prescribes to- but it is helpful to have other women who are nursing longer than just 6 months.
I highly recommend joining the La Leche Roselle online- it is mostly for chat and you can e-mail your questions as you have them for nursing. They are a GREAT help for any nursing questions and can help you better than anyone. VERY helpful resource. I joined a few years ago- you can put it on a daily digest so it doesn't e-mail you much. You will get updates as to meetings (I no longer attend and that is ok). But it is good to have other nursing moms to help you and guide you through this.
La Leche Roselle (you can just chat on line with any questions you have- but must join online group first).
Send e-mail to: ____@____.com
Attachment Parenting (and this may or may not fit your beliefs but the online help is great):
Send e-mail to: ____@____.com