Snowballing?

[Jose]

hAVE U EVER HEARD OF IT OR DONE IT MY BUDDY WHO IS Bi SAYS HES DONE IT WITH BOTH A GUY AND A GIRL AFTER A BJ THEY KISS AND PASS CUM BACK AND FORTH

[Scarface]

That is usually called "cumswapping" though this type of question isn't allowed because it's not a sex education related subject. though to answer your question in the simplest terms. it's common for couples to do this because it's a very intimate thing to do..