Let both burn white.
From what I've heard on EWTN, here is how it works.
Two votes in the morning.
Two votes in the evening.
If the Pontiff is elected in the first ballot (morning or evening) then the result is announced right away (which means that we are now on the third ballot of the conclave, second of the day). If the Pontiff is elected on the second ballot (morning or evening) then both groups of ballots are burned white.
If no Pontiff is elected on both ballots of any one voting period, they burn black.
Are you saying that we are already on the third vote for today?