There are a lot of ways to skin this cat. Using two liners and temporary pins is easiest, just use both liners, and squeeze the pin area tight with your fingers, or use a clamp/vice grips. However, if you make nail-breakers (heavy pulls), with thin liners, you can distort your holes this way, inducing slop into the fit up later, especially if you don't have both liners tight to the spring, with precise holes.
Whether this is an issue, depends entirely on the type of slipjoint you make, and how you make it. If you're making all mono-steel knives where you nail everything together before you grind the blade, no big deal. If like me, you use mostly damascus, utilize a lot of exotic materials, trying to make them as thin and slick as possible, and need to have every component completely finished before you nail it together as the final step, it's can be a problem.
Of course, I don't make nail-breakers, so NBD. I do however take mine apart probably 100 times before one like that is finished, so I like temp pins with little handles on them so i can disassemble and reassemble quickly. Each one is a little different from the next or previous, so making a hard jig isn't viable. I used to use a rise/fall/ruple style jig but found it too be too time consuming to monkey with for my methods.
So I just use my liners and temp pins as mentioned above.
One good trick though, if you do like heavy springs, and need something solid to get things dialed in with. Take your liner, and transfer the holes to a piece of hard wood. Use short capped temp pins that fit tightly and use the wood as your jig. Easy enough to use the same piece of wood dozens of times, marking which holes are which if you're not working off a precise pattern.
IMO, I never saw the need for hard pins for the spring pins when doing fitment, although for the pivot you do want a precise, hard, and smooth finished temp pin, such as a bright finish drill bit. For spring pins though, I just use 1/16 tig rod that I cut about 1.25" long and bend into an "L" shape, using the short leg for a handle. Chamfer the ends a bit with a cup burr to make it easy to insert and remove. The pivot temp pin on the other hand, grind to a soft point, so that you can insert it easier with pre-loaded springs under tension.
