One or more of your assumptions is not correct.
You should have \$\theta_{ja} = \theta_{jc} + \theta_{ca}\$, giving \$\theta_{ca}\$ of 30.76 C/W for IC1 and 21.76 C/W for IC2.
With those values, IC1 should have a case temperature of
23 C + (1.2 W) (30.76 C/W) = 59.9 C,
which is pretty close to what you measured; but IC2 should have
23 C + (0.7 W) (38.23 C/W) = 38.23 C,
which is way off the measurement.
A likely cause are if the datasheet's \$\theta_{ja}\$ value makes some assumption about board layout that is not met in your application. If your board doesn't provide sufficient copper area attached to the chip to spread heat away, \$\theta_{ja}\$ will increase substantially. It's also possible that the datasheet value assumes some forced air cooling over the chip.
Balco is 70% Ni and 30% Fe. The alloy has a temperature coefficient of resistance of 0.00518, with better linearity than pure Ni. I don't know what your alloy is, but consider that a starting point.
Best Answer
They represent two different things:
Those two factors are not correlated to each other, as each one depends on the material and represent a different property.