It’s very easy actually. The key goes in the tabernacle before the first of 4 masses in a row. Sometimes the deacon, sometimes the priest counts hosts before the first mass. During the mass, sometimes the priest sometimes the deacon goes to the tabernacle to put the hosts back after communion. After mass, sometimes the priest, sometimes the deacon (or a different upcoming deacon) goes to the tabernacle to count hosts once again (our pastor is adamant about communion count at each mass). Tossing back and forth the key would be ridiculous, unnecessary, and silly. It would cause several unnecessary trips back and forth between the sacristy and sanctuary to find out who has the key to get it— with not a lot of time in between masses and many other tasks to take care of in between. Sometimes the priest runs back to the rectory to get a bite, or 5 minutes by himself, or take a break while the other priest is celebrating mass. How many times a priest would absent mindedly keep it in his pocket, therefore there being a huge problem during mass when the priest needs to put the hosts back in the tabernacle, but can’t because the key isn’t there.
I dont’ understand why this is so hard to understand, or why this long explanation was even necessary.