Actually having a second motor to recharge the battery would be very counterproductive. If you think about it, what is powering the 2nd motor to charge the battery? the same battery that it's draining? and even if you can manage to setup the 2nd motor to activate on downhills only your bike would probably slow down considerably with all that friction 2 motors would have. The 2nd motor would be highly detrimental for your uphill and flat land riding also with the extra weight = more battery power needed to get it going.

If you really want "some" form of charge on your battery then the best bet would be to attach 1 or 2 of those 12v car solar panels on your rear rack and wire it to your battery. At least this would minimize the friction (panels are lighter than a motor anyday and are not on your drive train)