Quick weight question for an upcoming trip

[Nanno]

Unfortunately the timing means this would likely be one of the first major hikes of the year--the Sierra is too snowpacked to have allowed for much hiking yet this year. Might have to prioritize a fitness regemin.

If you want to make this trip a "go" this summer and have fun, then it's time to start working out now and plan a program through the next couple months. You don't have to hike in the mountains to get your boys in shape. We've often done most of our conditioning on quiet roads around neighborhoods. Heck, I used to ride my bike on dirt roads and have my goat, Cuzco, run along with me. Your goats should be out walking with you every day, and for conditioning purposes I'd say at least three miles a day of brisk walking and some jogging to help get them in shape, plus a longer walk once a week. It will be worth it. There's nothing less fun than getting three days into your planned hike and discovering that your goats are not up to the task because they've been out-of-shape "pasture potatoes". 

I assume this is without the pad? Good advice, I'll try this. 

Yes, always try saddle fit without a pad first. Set it up on the withers and slide it back until it "sets" behind the shoulder. This is called the "saddle seat". Young goats' saddle seats aren't usually well-defined yet, but you should still be able to get it. Check that your saddle looks level front-to-back and rock it back and forth to get a feel for how it sits. It should want to stay in place--not rock back and forth. Make sure the front forks are not riding down on your goat's spine (saddle too wide) and that your saddle doesn't look "perched" on your goat's back (saddle too narrow). Now crouch down look at it from the back. Does the angle of the tree match the angle of your goat's back all the way down the panels? Run your hand under the saddle on both sides and feel for pressure points, gaps, and bridging. No saddle is ever 100% perfect because saddle trees are static and goats are dynamic, but it should look and feel like it fits the size and shape of your goat's back. 
If your saddle looks good without the pad, add the pad and check again. A too-wide saddle can usually be fixed with a thick enough pad, but a too-narrow saddle cannot. Think of it like shoes--properly fitting shoes are ideal, but if they're a little too big you can add an extra pair of socks and make them work. But if your shoes are too tight, adding more socks might pad your feet for a short while, but in the long run it's only making the problem worse. One thing about pads is that because they are thick and rigid, they actually do add length to the saddle. Your saddle may fit fine without the pad, but if the pad hits your goat's hipbones it will still knock the saddle forward with each step. A pocket pad is probably the worst choice in this scenario because it can't be positioned differently. A regular non-pocket pad could be moved further forward onto the shoulders as long as the saddle itself is still positioned correctly. This is not ideal, but it's better than the alternative.   

I think I can take measures to reduce pack weight by increasing resupply frequency. This should drop the weight to 15 lbs--closer to 10% of their body weight. Then I can trade the 3lb. bear can back and forth, or maybe just carry it myself. 

I think you might still do better to go with one pack and load it heavier than to try to pack both goats, especially if there's even the slightest doubt about conditioning or saddle fit. 

I've been a bit worried about that, but it's damn hard to find anything like an "expected weight" chart or similar. Does ~63kg seem outside the healthy range for Alpine/Toggenburg cross of that age, or just on the small end of healthy?

I'd say yours are at the small end of healthy, but it doesn't mean they won't catch up. My boys were about 125# at 1 year and about 150# at 18 months. At two years they were around 180# or more. However, mine were May/June babies, so they'd already had that spring growth under their belts by the time they turned two. Mine are also half Nubian and they tend to grow bigger than Alpines and Toggs. Mine have topped out at around 195# and 220# at nearly 4 years old. A friend of mine has an Alpine/LaMancha cross that just turned two years old and I want to say he's over 160# just by looking at him and he was a December baby. So yeah, your guys could definitely use some more growth. Good eats and good exercise will help.