What's the name for a play on words where two words together sound like a longer single word?

Solution 1:

That sort of play on words is often called an oronym.

Homophones of multiple words or phrases (as sometimes seen in word games) are also known as "oronyms". This term was coined by Gyles Brandreth and first published in his book The Joy of Lex (1980), and it was used in the BBC programme Never Mind the Full Stops, which also featured Brandreth as a guest.

Examples of "oronyms" (which may only be true homophones in certain dialects of English) include:

"ice cream" vs. "I scream" (as in the popular song "I scream. You scream. We all scream for ice cream.")

"euthanasia" vs. "Youth in Asia"

"depend" vs. "deep end"

"Gemini" vs. "Jim and I" vs. "Jem in eye"

"the sky" vs. "this guy" (most notably as a mondegreen in Purple Haze by Jimi Hendrix)

"four candles" vs. "fork handles"

However, as a caveat, oronym is not an antonym for portmanteau. It just refers to words or phrases that sound the same, and the meaning goes both ways. But it is a term for that sort of play on words.