who ever is receiving the in game bells sets up items in retail for 999,999 each (highest) like flowers, seashells, items, whatever you are okay with giving away.
then the other person fills all pocket slots with bells and all empty letter slots with bells so in one trip to retail you can buy two items for 999,999 which is essentially 2M. just a few trips back and forth from the atm and retail and you'll reach the amount much faster than individually dropping each bell bag.
the host just needs to keep replenishing the retail items and keeping track until you reach desired amount. the bells will then go straight to your atm the next day I believe (maybe earlier I forget).