Why not just create a new AD group and add the 1-2 users to it? Then map the the new group to the elevated role and you're done.
The right solution would be (as mentioned before) to assign these users to a specific LDAP group, and map this group to the needed role in Splunk.
However, if this is not a solution for you, it is still possible to create dedicated users locally on the Splunk Search Heads for these users. If it is only for one or two users, this might be the least complicated way. You have to be aware, that you have to manually manage these user accounts then, if anything changes in between.