What are you using? If you're using ArcMap 10, are you using Data Driven Pages or MPS Atlas?
If you're using Data Driven Pages (which is core functionality of ArcMap10), you are out of luck. Data Driven Pages only allows for one data frame to update dynamically with the current page. It is explained at the ArcGIS10 Resource Center. Read the Detail Data Frame section. Also, the ArcGIS 10 help within ArcMap specifically says that only one data frame can be linked.
If you're using the MPS Atlas extension (on 9.3 or 10) you can have your mapbook link to different data frames through the Data Frame Manager in the MPS Atlas toolbox. To my knowledge, MPS Atlas is what you'll need to solve your problem.
To have 2 sheets per layout page (and if you want to avoid using scripts), you'll most likely need the *Production Mapping toolset (which includes MPS Atlas). It provides a large degree of cartographic customization. Much of it you likely won't need, but it is essential if you want anything greater than the basic mapbook functionality in ArcMap 10.
*If you're using ArcMap 9.3, it is called PLTS.