I get the distinct feeling that "inbetween" occurs increasingly often as a single word, but I'm not at all clear on why it's used more in some contexts than others.
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I had not previously been aware of seeing it printed other than as two words, but the practice seems not to be particularly new. The OED records the hyphenated noun in-between as meaning ‘(a) An interval. (b) A person who intervenes.’ The first citation is dated 1815:
It’s followed a year later in Jane Austen’s ‘Emma’ by:
As an adjective meaning ‘placed between’, it occurs first, once again hyphenated, in 1898: