Power management IC selection based on two independent batteries

I'm trying to construct a two battery system that powers an electronic device.  The current electronic device acts as the SMBus master/host for the smart battery designed for it. My two independent system must interface with the device's SMBuss host.  The system needs to be able switch between batteries (can't operate in parallel). The issue that I'm facing is selecting the appropriate Fuel guage.  I'm not allowed to access the device's firmware to determine which Smart Battery command protocols are being relayed (I do realize this might present problems later on.)  Is there a fuel gauge that will allow me to monitor two batteries at the same time while only having one batterys properties relayed to the SMbus. If this is not possible, is there a fuel gauge (2 fuel gauges) that I can use for both batteries that can be turned on and off to add and remove it's address from the bus for use with the electronic device?  I'm still learning about SMbus. Is it possible to disconnect  a smart battery and add a smart battery without creating an error with the master/host?  Right now I'm just trying to figure out the time for implementation fo the project so any help would be appreciated. There is still a huge learning curve.