One device period.
You cannot have 2 ASIO drivers running.
But
It is not a problem to hot swap interfaces, I used to do it all the time. Sonar will first warn you that you have unpluged device #1, then when you plug in device #2 ask if you wish to use it.
It's just not possible with a PCI card, only USB ( firewire?)
So you can use 2 interfaces, you just cannot use them at the same time.