But it is true that the OP has opted for 60m of cord in the second's pack, that's what his question is about, and recommending a ``better way'' isn't responding to the question, its changing it to one you prefer to answer.

There's no way I'd ever carry a 5.5mm 60m cord in the second's pack for rappelling

The OP isn't thinking about packs that big, he's talking about the BD Magnum, a 16L pack the same size as the Bullet. If the Bullet is really too small, the Cilogear 20L sounds like the best option to me. (You can add compression straps to tamp it down too.)

Why so? Most of the load and friction is on the 10.5mm line. Has anyone come to the conclusion that it's an inefficient/unsafe system? What do you use?

Ok, you did ask, so please don't blame me for OT ranting...here are a few things that occur to me:

1. The difference in slip rates is a very serious problem solved by rigging the rappel so that the climbing rope passes through the rap rings and the joining knot jams against the rap rings, preventing the climbing rope from being pulled. This means the rappel must be pulled with the 5.5mm cord. This will be a very nasty experience with wet ropes and a hard-to-pull rappel.

2. Speaking of hard-to-pull rappels, the knot can actually jam in the rings.

3. A 5.5mm rope can tangle horrendously, and serious amounts of time can be consumed undoing the messes. Not good when the sun is setting.

4. If the rappel hangs irretreviably after one end is up and out of reach, you are left with the option of either prussiking on 5.5mm cord (of course, not being sure whether the rope will hold---this would be very dicey in any case, but it certainly has been done), leading back up on the 5.5mm rope using the ``inchworm'' technique on the accumulated slack, or cutting off whatever 5.5mm cord you have and using that for the rest of the descent. In all cases, you have to do your business on a 5.5mm static line. Frightening.

5. You are essentially doing single-strand rappels. Forgetting to rig your descending devices appropriately (double biners?) could lead to a very rapid descent.

6. Putting a full-length rope in the second's pack is a waste of resources. There are all kinds of uses for such a rope during the ascent if it is running from leader to second.

In my opinion, the smallest line one should consider for a trail line is 7mm. Tangling with it is plenty bad enough. As for what I do, if the climb is primarily a free climb (no jumaring), then I prefer half ropes for their incredible versatility and extra safety margin. However, there is a weight penalty, which perhaps could be offset by paring down that 30 pound rack!

I'm confused and intrigued by your description of your system.

could you explain how you rig raps a bit more? what's an 8 plate? is it like a figure 8?

when you rap does the knot rest on the rap rings/bolt hangers?

I read something in your post about fixing the tech cord to the next rap station for when the second raps, is this just to keep from pulling the knot against the rap rings more than necessary?

I'm kind of interested in your system as an emergency/bail system, but I can't quite figure it out from your post.

Get the Cilogear 20L. I got mine this August, and have used it doing multipitch trad, dayhikes, mountain running, and mountain biking. it is a great size for these, and mixes durability and lightweight really well. I've led some easy chimney and offwidth routes with it on, and it doesn't even show. The price is right, the company is great, and it clips easily onto my Cilogear 60L.
now get it already and go climbing!

So I need a small pack so my second can climb with our approach shoes, water, and 60m of 5mm Tech-Cord.

How does your second feel about this? And shouldn't they be the one picking the pack?

Some of the packs mentioned differ considerably.

Ozone: weight 36 oz, volume 2300 cc.
CiloGear 20L: weight 10 oz, volume 1220 cc.
Bullet: weight 13 oz, volume 976 cc.