Build a setup where a Hopper Minecart is sucking out a container, and a Hopper and another Hopper Minecart sucking out the upper Hopper Minecart (check out the first attached document).

If you put an item in the upper Hopper Minecart, the item will go to the lower Hopper Minecart whereas if you put the item into the container, the item will go to the lower Hopper.

I expect it to be reliable, either by going to the Hopper Minecart or the Hopper.