What you do is have participants state whether they want to enter a ticket for 1 spot or 2 spots.
At the drawing, if a 2 spot ticket is drawn, the game is over. If a 1 spot ticket is drawn, draw more tickets until another 1 spot ticket is drawn.
Would have to come up with a model to fairly price the tickets but I'm thinking the relative price of the 1 vs 2 spot tickets would be dependent on their distribution. You'd also have to assign the relative values of the prizes. 2 spots is more than 2x the value of 1 spot, correct?