Is it possible to use two of cellular interface as primary?
Primary and backup use is not possible as far as I searched in Cisco.com.
I don't have two HWIC so can't confirm.
Cisco 3G WWAN HWIC Q&A Technical
Features and Functions
Can I configure the 3G HWIC WWAN cellular interface as the primary interface and
configure another interface as a backup to the cellular interface?
No. The 3G HWIC interface is an asynchronous serial, DDR interface.
You cannot configure another interface as the backup interface to any DDR interface
because the DDR interfaces themselves are designed to function primarily as backup interfaces.
Any reply will be apreciated!