Impossible. In order for an InstantX transaction to be locked, a majority of the eligible masternodes for that UTXO must sign the lock. Thus there are not enough remaining eligible masternodes to sign the same UTXO again. (If Dash doesn't actually work this way I described, then it should be fixed to work this way.)

Afaics, what you are describing can only be done at the juncture where the deterministic eligibility calculation is updating (every N blocks or so). I had already written upthread about the flaw that is the ambiguity due to competing orphaned chains that straddle that eligibility update.