There is nothing that prevents someone from specifying multiple protection levels for the same piece of equipment. The trick is to understand EXACTLY what those protections mean, and then conform to the most stringent possibility.

Paraphrasing: Ex ia means the exposed machine surfaces (internal and external) will never get hot enough to initiate a flame (and hence, no explosion), nor will it create an inadvertent spark to permit ignition. Ex d means that once an ignition occurs, the flame path is sufficient to keep the (internal) flame from reaching the surrounding (outside) medium. Which is the more restrictive? Depends on materials and geometry, to some extent.

My "gut feel" is that if you meet Ex ia, you don't need to meet Ex d because there won't be an ignition to create a flame for the path to squelch.

Converting energy to motion for more than half a century

The following may be of interest, though I may not have completely followed:

AS/NZS "IEC" 60079 Cl 16.2.2.5.2:

Conductors of intrinsically safe circuits, except as permitted in 16.2.2.7, shall not be in the
same bundle or duct as conductors of circuits which are not intrinsically safe unless separated
by an intermediate layer of insulating material or by an earthed metal partition. No separation
is required if metal sheaths or screens are used for the intrinsically safe circuits or the circuits
which are not intrinsically safe.