When 2 minecarts with hoppers that are 1 Y level different but on parallel tracks pass by one another the upper hopper cart passes items to the lower hopper cart.

For reference, this would be a typical setup for a basic auto smelter. About 50% of the smelting items are ending up in the fuel hoppers that feed the furnaces.

I locked the upper hoppers and I was able to observe items passing to the lower minecart despite the lower minecart being -1x, -1y, 0z offset from the upper cart.