In our environment, we have a general Active Directory lookup table in SQL that we primarily use for filling in metadata like this. This guide may help you get something similar set up: Querying Active Directory Data from SQL Server (mssqltips.com).

When referring to a domain user, the LFDS/Forms username is formatted as:

<DOMAIN>\<USERNAME>

In our AD lookup table, we have added a computed "SamFormatted" column that concatenates "<DOMAIN>\", SamAccountName, so that we can easily use this value for task assignments.

In your situation, I'd create lookup rules that fill your "Attorney-in-Fact 1" and "Attorney-in-Fact 2" dropdowns with the "SamAccountName" column (possibly filtered where job title matches "Attorney-in-Fact", or however you have your AD setup to limit the selections to valid users). Then add lookup rules that, when "Attorney-in-Fact 1" and "Attorney-in-Fact 2" dropdowns match the "SamAccountName" column, fill the respective "Attorney-in-Fact 1" and "Attorney-in-Fact 2" name fields with the "Name" column (and fill hidden SamFormatted fields with the "SamFormatted" column, if you're using their LFDS username for task assignments). This would require 2 rules per Attorney-in-Fact selection, so 4 rules total.