We do not currently know of any synonyms for toss-up question.
The noun toss-up question is defined as:
During a classroom quiz or game show, a question posed to two opponents where both are able to answer by signaling, and depending on the rules, the one that responds with a correct answer either takes control of the game or gets to advance to a subsequent round.
New Story Writing Game!
Win Amazon gift cards by writing simple stories at Fictish,
a new story writing game backed by WordHippo.