If Exist or Exists?
If SQL Server
IF EXISTS (SELECT * FROM tblOne WHERE field1 = @parm1 AND field2 = @parm2) OR EXISTS (SELECT * FROM tblTwo WHERE field1 = @parm5 AND field2 = @parm3) PRINT 'YES' Is fine, note the only thing changed is EXISTS not EXIST. The plan for this will probably be a UNION ALL that short circuits if the first one tested is true.
You could also write an IN statement
IF EXISTS (SELECT * FROM tblOne WHERE field1 = @parm1 AND field2 IN (@parm2,@parm3)