UNION 2 Consultas de selección con columnas calculadas

Creo que la idea clave es obtener la lista de impresoras que desea:

select PrinterGUID
from ((select cm.PrinterGUID
       from computermapping cm
       where cm.ComputerGUID = '5bec3779-b002-46ba-97c4-19158c13001f'
      ) union -- to remove duplicates
      (select PrinterGUID
       from usermapping um
       where um.UserGUID = 'd3cf699b-8d71-4dbc-92f3-402950042054'
      )
     ) p;

Entonces puedes usar exists para ver si estos son los valores predeterminados:

select p.PrinterGUID,
       (exists (select 1
                from computerdefaultprinter cdp
                where cdp.PrinterGUID = p.PrinterGUID and
                      cdp.ComputerGUID = '5bec3779-b002-46ba-97c4-19158c13001f'
               )
       ) as is_computer_default,
       (exists (select 1
                from userdefaultprinter udp
                where udp.PrinterGUID = p.PrinterGUID AND
                      udp.UserGUID = 'd3cf699b-8d71-4dbc-92f3-402950042054'
               )
      ) as is_user_default
from ((select cm.PrinterGUID
       from computermapping cm
       where cm.ComputerGUID = '5bec3779-b002-46ba-97c4-19158c13001f'
      ) union -- to remove duplicates
      (select PrinterGUID
       from usermapping um
       where um.UserGUID = 'd3cf699b-8d71-4dbc-92f3-402950042054'
      )
     ) p;

Aquí es un db<>violín.