Let's say a character/race has explicit statements for both HDE and BDE (Type 2). Are there explicit standards saying you have to choose one or the other, or can you just index both and shrug at the contradiction of that?
That depends on the context itself, and hat type of BDE.
But since the OP mentions BDE2, yes, you can have them simultaneously, but only[and only] in very specific context.
For example, let's say Realm A sees Realm B as fiction. That warrants that Realm A is 1-A [assuming Realm B is of the standard notion of dimensionality]. However, regardless of the R>F itself, Realm A can have its own higher notion of Dimensionality just like Realm B.
In such a case, a being that is from a higher dimension of Realm A can get BDE2
with respect to Realm B and HDE
with respect to Realm A's baseline 3 dimensions, thus technically having both BDE and HDE at the same time.
Although you won't get any
qualitative[based on quality/composites] layers above baseline 1-A simply by the higher reality level having its own dimensional hierarchy, it can still be
quantitatively/based on Quantity] higher.
However, all this is, a specific example. In general, it really depends on
how the BDE2 realm got their BDE2/1-A rating. For example, if Realm A and B had R>F, yet realm B saw Realm A as a higher dimension that could be reached via stacking quantities, then that would be a disqualifier for BDE2 instead, disregarding their 1-A rating entirely.
You can see
this thread to confirm that what I said is backed up by staff's approval.