This short story is unmistakably Now Showing at the Roxy by Harry E. Turner, first published in 1977. The story deals with two competing cinema owners: Stan Rabble the proprietor of The Roxy, and Lou Rouser who owned The Luxor.

As their rivalry grew, they started to incorporate real elements into the screenings, for example:

Only at the Roxy, his advertising blurb read, Will a Genuine Rapist
Mingle with the Audience and Savagely Assault Defenceless Women while
they Innocently Enjoy:

BLUEBEARD’S JOURNEY INTO THE INTESTINES OF A WHALE

When this combination proved highly popular, Rouser hit back by screening a double-bill of The Sound of Music with Mary Poppins, with the added element of the audience taking a one-way escalator trip into a lake of concentrated sulphuric acid:

‘Surprisingly enough,’ he said in a chatty, conversational tone, ‘the
escalators weren’t so expensive to install. But, Stan old pal, two
hundred thousand gallons of sulphuric acid was a real hefty investment
— even at wholesale prices.’

According to the ISFDB this short story has only been included in two anthologies, The Tenth Fontana Book of Great Horror Stories and 65 Great Tales of Horror, both edited by Mary Danby. Presumably one of these is the collection sought by the OP.