[Samba] strategies to run two NT4 domains or merge them on one samba host

[Steffen]

Hi,

we currently run one samba v3.0 domain "DOMAIN30" with WinXP domain 
members and Win7/8 accessing the file server without domain membership.

Then we run a second samba v3.5 domain "NEWDOMAIN" with WinXP/7/8 domain 
members. Which was migrated from a NT4 PDC to samba 3.0 and to v3.5 
eventually.

Neither domain has anything fancy about it: users and joined workstations, 
but no trusts, almost no groups (could be re-created manually easily).

a) I want to upgrade at least DOMAIN30 to a samba v3.5. And preferrably 
have one user base and one domain NEWDOMAIN.

https://www.samba.org/samba/docs/man/Samba-Guide/ntmigration.html

explains "If you want to merge multiple NT4 domain account databases into 
one Samba domain", that I can merge user and machine accounts from 
different domains into one, right?
But how can I make existing machine joined to DOMAIN30 to contact 
NEWDOMAIN? Is is possible to merge two NT4 domains into one samba 3.5 and 
keep all joined members?
Can I run a second smbd on the same server with another domain name that 
forwards any auth request to NEWDOMAIN? Can I do something with aliases?

b) Would a migration of both NT4 domains to samba 4 help?
So I would merge the users only, create two NT4 domains in the AD and 
leave the machines in there?
New machines would joine NEWDOMAIN only and eventually DOMAIN30 dies 
because of lack of members.
Is there a documentation how to migrate two or more NT4 domains to smaba 
4?

Kind regards,

-- 
Steffen