That's why I am hoping I can just add a dongle like this:
http://www.amazon.com/IOGEAR-Blueto.../dp/B007GFX0PY/ref=cm_cr_pr_bdcrb_top?ie=UTF8
One problem I foresee is that previous OS X systems only allowed one Bluetooth host device. I was able to use an app called Bluetooth Explorer, that allowed me to select between the internal and USB Bluetooth. The downside was that after repairing the (old) keyboard/trackpad to the USB Bluetooth I lost the ability to do the boot key presses such as option to select between OS X & boot camp, or enable recovery mode. I assume this was due to lack of firmware support before OS X loaded?
To be fair, I have not played with it recently to troubleshoot the USB dongle further.
Maybe this has changed in El Capitan to allow two active Bluetooth dongles at the same time? Probably not, but I have not tested.
Ideal setup for me would be to have the internal Bluetooth run the keyboard (to keep boot key presses active), and the external dongle run the new magic trackpad to add the force touch features, if possible.