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.