All right!!
The big problem that I had with my split-scene scenario was simple: Having enough figures/markers to use for the attackers. I ended up buying some rolls of pennies and nickels, and using most of my d6s (sorted by color).
So far, so good, though my bad guys haven't been able to get more than the most momentary foothold on the wall.

Too many fall to their deaths.) Meanwhile, the pcs are able to launch catapult missiles at my scorpion units, and they keep knocking down my siege towers before I can get inside the catapults' minimum range.
The defenders have a clear advantage; even though they are outnumbered, they're inflicting significant losses on the baddies.
The ogres under the mantlet are approaching the gates, though...
One of the most important elements of this battle is the abstraction of time. The two simultaneous scales of battle are on the same initiative set, but the unit-scale battle rounds are more like 1 minute long. The battle is an abstract representation of the battles for the walls over the course of several years.
Things are going quite well.

Oh, and we had six of the players present this time.