If both parties have equal amount of collateral tied up you could force them to settle quicker by applying demurrage to the collateral. The longer they wait the less they will get back at the time where they close the bet position. Demurrage rate could be applied in different proportion based on the VWAP of the last price on the market. Since all the honest betters will have settled earlier the VWAP will be difficult to manipulate. For instance, if demurrage rate is 2% per day and the vwap is 0.9, the loser will bear 0.9*2=1.8% demurrage whereas the winner will bear only 0.1*2=0.2% demurrage. In this condition, the loser is better off settling quickly.
Whatever the specific way this is inplemented, the counter-incentive of waiting should always be higher than the incentive of waiting so that the equilibrium will form on (tell the truth,settle early).