I was wondering how other enter contacts with different last names?

Right now, I enter spouses with the same last name like this:
Last Name: Smith
First Name: John & Jane
So the name displays as: John & Jane Smith

However, this does not work for couples who do not share the same last name.

I thought about using the Spouse/Partner field under the details section.
However, to make this a viable option, I need to be able to move that from
the Personal Information Group to the Business Contact group in the Customize
Form option. Is this possible?

Does anyone have any better/other solutions?

I do a lot of mail merges, so the solution must be able to display the full
name of both contacts on letters/envelopes. First names only would be a plus
too for letters.

#For a full answer i would need to know what sort of mailings you send out
but from my work dealing with about 3000 contact I have, a full mailing of
general information on A4 sheet folded once (easier to put into envelope
taking into accout UK postal rates on size of envelope.) with a sticky
address label attached.
For individual letters what about window envelope rather than trying to
match up address printed envelopes with the customized letter.
How you hold the data? Well there are several ways , the easiest is to use 4
fields slightly different from their original purpose like you do for first
name.

field 1 john
field 2 SMITH
field 3 jane
field 4 JONES
(
That shouls give you
Dear & (possible for some condition if field 3 was blank!