i would worry that the clip would interfere with the release of the cleat
as inconsistent release is
imho
the main problem with toe clips

also i cannot imagine why you would want a back up for your clipless pedals
if you often experience unplanned release from clipless pedlas
then there is something else going on

however
you could put the toe clip on the opposite side of the pedal
and just keep the strap snugged down to keep the clip from hitting the ground