Wirelessly posted (8703e Sprint: BlackBerry8703e/4.1.0 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/104)
By the way, RIM has done somewhat of a dual core processor before. The Nextel 7510 abd 7520 have 2 processors. 1 for the radio (Walkie Talkie, Phone, and Data transmission), and a separate processor board for the OS/PIM. The reason for this is because (among others) the processing required to run the walie talkie and packet data on iDEN is nore intense than other platforms (lower latency).
The downside as you have already mentioned is battery life. In fact, if you place a Nextel BB side-by-side with another carrier's BB and do not even use them, the Nextel BB's battery will die noticably faster than the other.
I can tell you from experience, the Nextel BB does out-perform other BBs is processing speed and even at 56k data speeds it actually performs basic blackberry functions faster than even the high-speed EVDO devices (with the exception of web browsing and downloading apps).
The bummer is that the 7520 is end of life, and no iDEN BB is on the roadmap, not to mention that it does not support MultiMedia and all of the newer features.
The bottom line is that what you have suggested is more than possible, and RIM has already done it.