Our daughter was 2 1/2 when our twin boys were born and we use the Graco iMonitor Duo Baby and it's pretty good - we bought it on clearance, and I think the batteries weren't charged properly as they don't last long away from the base, but just bought replacement battery packs ($38 for two packs) and are making sure to charge each for 12 hours on the base before using it. The options were very limited, but we are happy with it. here is a link:
http://www.target.com/Graco-iMonitor-Duo-Baby-Monitor/dp/...
On another note, you could move the monitor out of your older child's room and just continue to use the one you have. We chose not to do this and have been glad as our older daughter has woke up sick in the middle of the night and we were able to hear her and get to her right away.
Good luck with the twins ! You are in for quite a ride!!!