Is it possible to define a double-character property?
No.

You are, after all, defining a character class. A character class almost always1 matches exactly one character, never taking context into account.

How might I handle this?
Define a rule, not a character class. Out of curiousity, which 'r' in 'rr' is the vowel? First one, second one, or both?

1The only exception I can think of are cases with case insensitive matching, where the Unicode definition defines that the "other case" of a character is a multi character sequence.

In reply to Re^3: Creating new character classes for foreign languages by JavaFan
in thread Creating new character classes for foreign languages by Polyglot

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