I definitely wouldn't do it. Those are different sizes and have different power requirements. So the t-stat will put out different amounts of power depending on which one the probe is over.

i.e. if you have the probe over the larger one, it will supply enough power to heat it, but that will be more power than is needed for the smaller one. Conversely the opposite would be true if you had the probe over the smaller one, it would supply less power and hte large one wouldn't get hot enough.

Get another t-stat, or if you're on a super budget or need it ASAP, make or buy a rheostat.