After this question really bothering me last night I had to sit down and do all the math and draw myself many pictures plugging in various resistances. After doing all this I have come to the conclusion that the rheostat in series after the proportional controller would work! I still do not believe it should be done though as it has the potential to become more probmatic than just using the proportional and will slow down the reaction time of the proportional controller possibly causing the temps to vary almost twice as much as it would with just the proportional controller. Here is the math that makes it work.

All numbers are made up just to use as an example.

UTH = Resistance of 100 Ohms

Rheostat = variable resistance of 0 to 200 Ohms, but for the primary example we will use a setting of 100 Ohms to start out.

Max Voltage that can be applied = 120 VAC

That means with just the UTH you would have a current of 1.2 Amps going through the UTH.
Amps = Voltage / Resistance
1.2 Amps = 120 VAC (Max AC voltage that can be applied) / 100 Ohms (UTH Resistance)
Thus max watts on UTH would be 144 Watts.
Watts = Volts X Amps
144 Watts = 120 Volts X 1.2 Amps


Add in the Rheostat set at 100 Ohms and the resistance of the circuit doubles to a total of 200 Ohms. Max current through the circuit gets cut in half to 0.6 Amps. Also the Max Voltage the UTH will see would be cut in half as each of the resistances would drop 1/2 the applied max current through it.
Amps = Volts / Total resistance of circuit
0.6 Amps = 120 Volts / 200 Ohms
Watts = Volts X Amps
72 Watts = 120 Volts X 0.6 Amps

Now in a series circuit the voltage drops across each of the resistances proportionally to the amount of the resistance. In this example the resistance are equal, so 60 Volts would be dropped across each load. Also in a series circuit the Current is a constant so each load will see the full 0.6 Amps of current flowing through it. This means each load will have to dissipate 36 Watts of energy.
Watts = Volts X Amps
36 Watts = 60 Volts (voltage used by each individual load) X 0.6 Amps

Now for the more advanced let us set the rheostat at 150 Ohms.
Amps = total applied Voltage (120 VAC) / total resistance of the circuit (100 Ohm for UTH + 150 Ohms for rheostat = 250 Ohms)
0.48 Amps = 120 VAC / 250 Ohms
Total Watts Used = Applied VAC X Amps
57.6 Watts = 120 VAC X 0.48 Amps

Now the voltage is not equally divided between the two loads since they have different resistances so we have to figure out what Voltage each load drops by using Proportions.

120 VAC / 250 Ohms = Y / 150 Ohms (Resistance of Rheostat)
0.48 Amps = Y / 150 Ohms
0.48 Amps X 150 Ohms = Y (We are back to basic Ohms Law)
72 VAC = Y
Now 72 VAC X 0.48 Amps = 34.56 Watts

Now lets do the UTH
120 VAC / 250 Ohms = Y / 100 Ohms (Resistance of UTH)
0.48 Amps = Y / 100 Ohms
0.48 Amps X 100 Ohms = Y
48 Volts = Y
Then 48 VAC X 0.48 Amps = 23.04 Watts

Lets double check.
Do the voltages dropped across each load equal the total applied voltage?
72 VAC (for Rheostat) + 48 VAC (For UTH) = 120 VAC
Does the total Watts used equal the Watts used by each load?
34.56 Watts + 23.04 Watts = 57.6

OK! I confess, I am an electroncis geek!