Andy wrote:
Is it possible to see that one is slotted and one round holed when the
cable is ready to be hooked up to the glider?

Easy: Pull at both ends. If everything is correct, then one link is now
fixed while the other can be moved freely.

And that's the idea of the arrangement: Over time, the weak link will
stretch, even when used within its load limit, and therefore weaken. In
other words, after some time, the link will be weaker than advertised.
The second link however will keep his strengh, as it never takes load.
So when the first link breaks prematurely, the second will still hold
the advertized load.

When you pull at both ends and one link can't be moved freely under the
load, then the first link has stretched and must be changed.

Stefan