Do you frequently have multiple clients purchasing a property together and need to enter, track, search, and merge their information?  You could add a second client contact using Premier customization features.

The best way to handle two clients would be to just add an additional contact for handling a second client.  After all, additional purchasers may not always share the same address information either, so this gives you all the same fields for entering both clients completely on your General tab for each job.  To set up a second client contact, use Maintain > Office Management Preferences, Contacts tab to set up an available contacts for "Client #2" or "Second Client".  Set the Role to the same one used by your main Client, so clients added in either place are easily searchable together for future use as clients.

Note that the order or position of the contacts as they are entered on the General tab doesn't matter, as that does not reflect the order or way items can be merged into other areas, reports, or documents.  One would not generally want to change the location of existing contact labels, since that would affect the label for that contact location on other existing jobs.  Unless another contact type has never been used for any prior jobs, then it might be switched into a Second Client.

To add the 2nd client to your invoice/receipt use the Customize > Customize Invoice Forms > Layout Designer to add the second client to your invoice form where desired to include its information.

Likewise for agreements, the fields for your additional contact are available for merging as well.  Use Insert > Office Management Field to include the appropriate second client fields as needed where desired.  If you'd like the client info alongside the first client, perhaps insert a table with two cells and include each set of merge fields in each.  If unused, the additional client info of course simply won't merge in.