Have only seen one response to this post so I thought I'd share my experience.
When my first child was 6 months old I too became pregnant.  I continued to bf
exclusively until 7-8 months when we started bananas, etc. At 10 months our
pediatrician did an iron test, baby was fine but she told me I could start
cows milk if I wanted (I didn't). By one year he was eating other solids and
still nursing several times a day even though there didn't seem to be much
milk.  When second child was born the first was 15 months, I tandem nursed til
my third was born two and half years later (oldest was four--now I'm nursing a
2 1/2 and a newborn). I found that nursing both at the same time helped solve
many jealousy issues.  The 15 month old almost quit eating when my milk came
in for the baby.  He loved the new supply but there was never an indication
that he would wean because I was pregnant (everyone told me he would). ( I
thought both would wean during my third pregnancy but they did not--they even
said the milk was different but tasted good.)

Just my opinion, but I think nursing two at a time made my mothering job a lot
easier.  This situation may not work for everyone, but I would encourage
giving it a try. Hope this post makes sense. I am writing in a hurry before
little ones wake up.