I think your assumption, that the rear wing is a little bit smaller than the on in front is not necessarily true.
There might be tandem wing planes with that constellation, but in some papers I found, the wing size differs between different models. The papers [1] and [2].
In addition some images depicting the mentioned:
In the case of the Miles M.35 Libellula, featured in the photographs above, the control surface arrangement was as follows: the front wing had flaps outboard and elevators (for pitch control) inboard. The rear wing had flaps inboard and ailerons (for roll control) outboard. Also, as can be clearly seen in the photos, the rear wing had vertical fins with rudders (for yaw control). (From page 18 of first cited source.)
If the front and rear wings have appreciably different span, it is logical to put the ailerons (roll control surfaces) on whichever wing has the larger span, and the M.35 followed this practice.
