lol. it depends.
if you hook them up in series, the total voltage will be double. if you hook them in parallel the available current will be double.
you should really match it to the motor. so if youve got a 3v motor and 2 1.5v solar cells then series is needed otherwise it wont go fast enough.
This would be the most important thing, seeing as its a race. you want highest power possible.
** if its a bright sunny day. wackem in series, itll make the motor go faster and there will be enough current as its nice and bright
** if its abit cloudy, put them in parallel. as you will need to compensate for the lack of light.
this is assuming that the output voltage of the cells is the rated input for the motor.
if you happen to know the votage on the cells, the current, and the voltage for the motor, let me know and i can help abit more.