Have a couple basic Q's on climbing-line (1 on safely descending/rappelling, another on using multiple lines and/or hardware to achieve "1+1=2"?)

Question

Hoping to ask a couple basic Q's before my order arrives, I suspect I may've gotten insufficient gear..

1 - Is doubling-up ropes, or hardware, ever acceptable in climbing? IE, my 8mm prusik cord is acceptable despite being below ANSI strength, because I'm attached to two tails (so it becomes above 24kN), does this same logic apply in ways like me taking (2) lengths of my climb-rope (Blue Moon), knotting each's end to an O-ring, and making my own cambium saver/friction saver? Or make a basal anchor by doubling-up in such a way?

2 - Is a figure-8 and your regular eye-in-eye hitch cord enough for a safe rappel? I ordered a harness/rope/etc but I only got ascenders, for descent all I got was a figure-8....if that & controlling the friction-hitch/eye-in-eye isn't sufficient for safe descent, what do I need?

Thanks a ton, am definitely going to be sure I never use anything not meeting ANSI but am planning on redundancies in some areas so wanna know these things, for instance if I set a canopy-anchor that my rope was running-through, but it wasn't a ring-in-ring but instead simply a carabiner tied to a lanyard, in such cases my instinct would be to just toss a 2nd carabiner there, even though the first one is ANSI-OK it seems to make sense both for peace-of-mind and just for rope-wear, heck I bought a handful of these steel thimbles to use in making loops on my rope, am already planning on using them in any&every instance they're applicable to reduce rope-on-rope friction!

Answer 1

Even though I am late on this, I'd like to try to give you some answers on your questions. I am volunteering in the german mountain rescue service, but I am in no way an expert in climbing security - so please take everything below with a grain of salt.

1.1 We use redundancies for virtually every type of rescue but there is usually a lot more weight involved (1-4 rescue personell + 1 patient + up to 100kg of equipment). However "redundancy" means two completly seperate systems: Two Ropes, two breaking mechanisms, two (or more) attachement points, etc. Using just a second carabiner would not qualify here, as you still have a single point of failure (e.g. the rope). Using only two ropes is even worse, as rope-on-rope is actively dangerous!
However: In four years of climbing, I have not seen a single person climbing recreationally with that kind of redundancy. Your hardware and rope usually have 22+ kN of strength (which is 2.2 metric TONS of static weight entry) - I am pretty sure that every bone in your body would shatter before your carabine does...

1.2 Is 1+1=2 applicable here? The easy answer is no. While it may depend on the specific case, you usually cannot ensure that both systems (ropes, hardware, etc.) have the weight (or impact) distributed equally between them. And if one of them snaps to beeing overburdend - the other one is sure to follow...

2 Yes, using a figure 8 carbine is a way to rappel (and was used here for many years!). We removed them in favor of the ATC-Guide (a tuber) onto which you can easily add a dead-man switch with a prusik. Just be sure to know how to properly use it.

TL;DR: I see no reason to use redundant systems in personal/recreational climbing. Get certified gear and learn how to use it properly. Take care of yourself. Be safe. Your life is not worth the savings on a cheap rope. And when in doubt, don't be afraid to ask the professionals (DAV in germany, UIAA internationally for example).
Lastly I think it is taking a look at the accident statistic of your local climbing association. There is rarely/never a case a rope just snaps - there usually is some kind of mishandling (or shard cliffedge!) involved. Reading through that shows you the common pitfalls - and hopefully makes you remember them! :)