I have the starting state:

  DB={}    Q={}

I would like to either remain in the starting state, or enter a new state:

  DB={Bob paid $15}  Q={Bob paid $15}

But this is Two Generals, which is impossible.

If you invoke 2PC, you want the states to progress thus:

  DB={locked}        Q={locked}

  DB={locked;        Q={locked;
      Bob paid $15}     Bob paid $15}

  DB={locked;        Q={locked;
      Bob paid $15;     Bob paid $15;
      unlocked}         unlocked}

A strictly harder problem, right?