SELECT M.msg_id, 
       SND.firstname as snd_fname, SND.lastname as snd_lname,
       REC.firstname as rec_fname, REC.lastname as rec_lname
from MSG as M
-- We'll use SND as the alias for the sender
join USERS as SND on M.msg_from = SND.usrid
-- and REC as the alias for the recipient
join USERS as REC on M.msg_to   = REC.usrid