In infopath, create 2 Secodary data connections to retrieve the data from each Sharepoint list
Add a First name field, Last name field and Combined name field, use the Dropdown control for the First and last name fields, connect the dropdowns to the appropriate data connections
Add a Rule to the Last Name field that sets the value of the Combined field to something like concat (substring(FirstName, 1,1), LastName)
Good luck
Dean