well, i arrived at a (partial?) solution. using some splitters, some logic bits, and a latch bit, this successfully steps through the 1st mini seq, then the 2nd, then back to the first, as desired.
the latch gets toggled every time the master seq. hits steps 1 and 5, and this allows the logic bits attached to the 2 mini seqs to gate the input to each mini seq, allowing each to run 4 steps at a time.
however, as you can see in the video, there is a bit of a problem with the master sequencer bit in this setup, it seems to not really provide a matching interval after each 4 steps (resulting in an unwanted "swing" effect --- deliberately adding a control for a swing would be nice, but here i really want this to have a consistent 8-step interval.) i don't know if it is because of the main sequencer itself or if there is some kind of latency introduced with the logic and latching. maybe someone has ideas on how to tighten this up.
check the video for my bit chain: