These pictures show how the flat belts thread on a Millbury
tenoner, to drive the top and bottom cope heads.  It is always
necessary to adjust and accomodate for the other belt somewhat
when both cope heads are in use.  Both the angle, and the height
of the bracket that carries the idlers will have a significant
effect on belt tracking.  The entire assembly with the vertical
posts also slides forward and back to adjust tension.  But with
2 belts running, sometimes one will have the correct height and
the other the best angle, in order that both run with adequate 
tension.  Both copes spindles are shown set up here for
demonstration; though the top is not carrying any cutting head.
In practice, if the spindles are not in use, take the belts off
which increases the safety of the machine and reduces wear, tear,
and noise