It isn't possible to jointly align the Archaea and Bacteria, but there's no reason that you can't create two separate databases to facilitate a join analysis. The scores will always be very low in Archaea but this will be interesting in and of itself. This might be as simple as adding and additional user variable "Archaea/Bacteria" to allow the user to specify which database they want to build/use.
It isn't possible to jointly align the Archaea and Bacteria, but there's no reason that you can't create two separate databases to facilitate a join analysis. The scores will always be very low in Archaea but this will be interesting in and of itself. This might be as simple as adding and additional user variable "Archaea/Bacteria" to allow the user to specify which database they want to build/use.