What's the proper voice leading procedure when two consecutive secondary dominant seventh chords (with the same root chord) occur before the resolution to the root chord? For example V7/ii -> V42/ii -> ii6. Is the V7 resolving into the V42, or is it thought of as a delayed resolution into the ii6? -Thanks!
I would consider this an extension of the dominant, not a resolution to an inversion of the dominant. Depending on everything that's happening, you may want to consider it all V42. If the dominant is spread over more than one measure, it would make sense to analyze it as V7/ii - V42/ii but if the bass note just moves to the 7 of the chord within a single measure, it may make more sense to consider the whole measure V42. Analysis can be somewhat subjective, so you may still analyze this within a single measure as V7 and V42 anyhow. It really just depends on the level of detail that you want to have and whether or not the additional detail helps bring anything important to your analysis. I would liken this to analyzing voices moving as individual chords vs ornamentation, where you could certainly analyze every note as a different chord but it doesn't necessarily better describe what is happening in the piece of music.
Call it a delayed resolution: you're going to exchange the root and seventh in a kind of wedge motion, because the 7th of the root position chord will need to move up to the root as the bass falls from root to seventh (in order to avoid doubling the seventh). That means there is no resolution at all for the root position chord.
In general, a change of inversion is just a prolongation of the chord.