The smoking of both herbal and resinous cannabis in the same pipe, chillum, or joint. So called due to the mixing of the Indian holy rivers of Ganga and Yamuna at Allahabad, and the similarity of the word Ganga to Ganja, denoting herbal cannabis.
Stoner 1: Oh my gosh I got some top Kerala Black here, got it off a rickshaw wallah in Mangalore, let us wrap up a spread.
Stoner 2: Naah man, you is crap. I am having in my possession some of the finest Manali Cream ever to grace the surface of the Earth, a gift from Shiva himself, passed on to me in a vision, and so we should do up a chillum.
Stoner 3: Let us compromise, have a melding of north and south Indian cultures, a Shiva-Parvati copulation of hash and weed: And do a Ganga Yamuna!
Stoners 1 and 2: Oh yea, oh yea, verrily, we shall conjoin the two forms of cannabis in one item, and enjoy their unique relationship, and verrily shall become mightily intoxicated.