Hosting 2 different domains

Question

we host our own exchange 2013 server internally with a couple hundred mailboxes for 'companyA.com'.
we are going to have a new startup company with only a handful of mailboxes/users (offsite) but they will be using
[email protected]
what is the best way to host 2 different domains on our existing hardware/licenses? 
is there a way to avoid having all of my existing users SMTP address appended with this second domain?  (if i remember correctly, when you add a new domain to the accepted domain list, all existing users get 1 SMTP address entry added to their list.
Thanks, Tony McPherson

Answer 1

if I add this new domain to the accepted domain list, my default policy says 'all recipient types'. but I don't see a way to distinguish
between companyA and companyB.
all of my existing users except maybe 1 or 2 are selected to 'automatically update via policy'.  so it seems to me that if I add the new domain name to the accepted domain list, they'd all get one new entry.  from the policy options page, I don't
see any selections that I could use to distinguish between the two companies.
*Specify the types of recipients this email address policy will apply to.
-All recipient types
or
-Only the following recipient types:
with options:
+Users with Exchange mailboxes
+Mail users with external email addresses
+Resource mailboxes
+Mail contacts with external email addresses
+Mail-enabled groups
Thanks, Tony McPherson