Here's as far as I got. I think it can be reduced to two equations, two unknowns, but it will run pages of derivation. I'd use a spreadsheet and cut-and-try until you find values that satisfy all conditions.
V1 = 12v, V2 = 5v, Vo = 3.3v. Each supply feeds a resistor, R1 for V1, R2 for V2. Both resistors go into the regulator input and supply currents of I1 and I2.
At one output current, probably the max value, you can choose the current sharing between V1 and V2.
R is the value of the internal regulator resistance which varies itself to meet demand. I don't care about its value, but it falls naturally out of the equations.
V1 = I1(R1 + R) + I2R + Vo
and
V2 = I2(R2 + R) + I1R + Vo
The regulator is modelled as a resistor, R, and a 3.30v perfect Zener, and R varies to always keep the current through the Zener above zero.
Power dissipation in the regulator and elsewhere is a separate calc.
There are limits to the values that the internal R can take on.
If the output current is 500 mA and the input voltage is 5v then R < (5-3.3)v/0.5A = <~3 ohms.
If the output I is 50 mA and the input voltage is 12v then R ~ (12-3.3)v/.05A = ~170 ohms.