Relation of case temperature to junction temperature
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.