Hey BCRider,
Wow, thanks a bunch for your response. There is a lot of information and thoughts there, and I know it took some considerable time investment to think and write that all out, and I appreciate your contribution. I just weighed the 30-30 and it is 7.2lb. I also weighed my unmodified Mark II G bolt-action .22LR (it has a scope mounted on it as well) and it weighs in at 7.2lb too. They both had their trigger-locks on for weighing, but I'd say that just accounts for the weight of ammo (they were both unloaded, duh haha), so we have a total of 14.4lb if the two were just mashed together as-is. Now, I'll lose some weight out of there by cutting off most of the buttstock of the .22, but I'll also gain some back with whatever mounting rig I come up with. So the rough overall weight I'm looking at with the current plan is probably about 13-13.5lb. For comparison, my .22-250 with the 26" heavy-barrel and scope comes in about 12lb. The big difference here is obviously that my .22-250 isn't exactly envisioned for humping around in the bush for 6 hours. The weight of the gun is the biggest outstanding challenge to overcome right now, I think.
The overall size of the combo gun isn't actually as bad as you might think. Check out these pictures of the layout of the 30-30 next to an air rifle I have laying around. I couldn't use my .22 because it has a scope mounted on it and that would mess up the spacing, but the air rifle measures almost exactly the same, so the layout is valid. After the first picture, I cover the air rifle's stock with a towel to hide the stock portion that I'm planning to chop off. As you can see, 26" OAL comes almost exactly to the back of the trigger guard. In the pictures, I pulled 30-30's box mag almost all the way out, to show the area I need to keep clear for it. The .22's trigger is not actually that far from the 30-30's. I can't say for sure until I lash them together and do some test-fitting as you say, but I think it will not be an issue to reach the second trigger. Now, the air rifle doesn't have a bolt, whereas the .22 I choose might, so I need to keep room open for that, but there's nothing saying I have to be able to remove the 30-30's box mag while the .22's bolt is back.
Since I don't really want to muck about with the .30-30 to try and cut its weight down at all (like drilling holes in the stock or anything), I'm left with a 7.2lb base plus whatever weight the .22 and mounts add. My Mark II (which weighed in at 7.2lb too) is listed by Savage as weighing 5.5lb naked. The Youth version of the same rifle weighs 5lb (and has a 2" shorter barrel, so it would only stick out 2" past the .30-30, bonus), but that still leaves a pretty heavy combo. A Savage Rascal weighs 2-5/8lb, but brings with it some other issues as well. It is only a single-shot rifle, which is not the end of the world but that means I'd have to walk around with it chambered if I don't want to miss quick opportunities on small game. And I'm not super keen on walking around in the bush with a round in the chamber; not really my style. Also, because the barrel is so short on the Rascal (and because I'm really trying to keep the OAL of each gun over 26", because classifying them as inseparable and thus a single gun over 26" long has me a bit nervous), I'd have to leave a big chunk of stock on it, moving the trigger far forward and putting me in that awkward shooting stance you mentioned. But on the other side it would lead to a combo gun that all together probably weighs only between 10-11lb... So right now I'm not sure of exactly my next step. I'll do a little more thinking and see what I come up with.