It isn't dependant on them being networked, stand alone is fine- you can download your avast program to each system or USB and use that to install, etc. - the multi user license can be applied to each PC (copy it to the other systems).
There isn't any automated way to merge them into one account, that would have to be done manually by one of the avast team. In all honesty I would choose one to be your default/main account email, this would possibly be best done with the account email address that you is used when logging in to the forums.
Unless there is some pressing issue not to use this email, it would save the grief of also changing the my.avast.com logon details, when you logon to the forums that is validated against your my.avast.com account.
So add/link the devices to this my.avast.com account and remove the devices from the other account.