Code requires 2 things: First that all junction boxes be accessible, and second that wire splices happen only inside junction boxes. That is why everyone is expecting this should be accessible.
So there are 2 big problems. First that breaker's handle-tie is completely hork-a-dork (credit for having a handle-tie, but they should've gotten a proper one). Either obtain the proper handle-tie for that breaker, or replace the 2 breakers with a quadplex 15-15-15.
The far bigger problem is this violates a cardinal rule: Neutrals and hots must travel together and Neutrals must serve only their partner hots. Any outlet must use the neutral from the same cable the hot came from. You can't loopy-doop neutrals like this for two reasons: Aside from current imbalance (which causes heating, this is AC power not DC!!!!) the big problem is we could overload the neutral. Imagine a 15A load plugged into the GFCI, so 15A on the red wire. No problem, right? Now imagine 2 boxes above it, 15A plugged in there. What's the current on the /3 white wire? You're darn right it's 30A!!!! So this won't do at all.
Now, a "CABLE" contains a number of WIRES inside a sheath. They don't sell cables with just a black wire, so there must be an undrawn white wire at top center.
Usually when "the last guy" does a dumb thing like this, it's to try to compensate for what is thought to be a broken wire. Often it turns out to not be a broken wire at all, but some sort of blunder. (remember anyone who does this is NOT the sharpest tool in the shed). Actual wire breaks are VERY rare.
So your task here is to complete the mapping of this circuit, account for every wire, and validate every wire to see what's really broken and what's merely a faulty wire connection at the device (notably: backstabs).
Then, you pick 1 place to break BOTH the black AND white wires: Just left of center where you did not draw a white wire, or above the GFCI. At that junction box, you disconnect one set and cap them off.
If all wires prove out good, and one of the possible break points is a 2-gang box with 2 yokes (4 sockets), feel free to feed one yoke with one circuit, and the other yoke with the other circuit. Do NOT split the 2 sockets on one yoke and feed 1 from each circuit (that would require something you can't do).
There's nothing abnormal about being afraid of going into an attic where blown insulation covers the rafters. When blown insulation covers the floor, you have no way of knowing if there's even a floor there. If there's no floor, then one misstep and your foot punches through the ceiling drywall and you have an injury, a fire department rescue, a BIG mess, and an expensive repair bill.
- The correct course of action is to get a whole bunch of trash bags, a quality mask and glasses, and shovel all the blown insulation into the trash bags until the floor is easily observable. Then create a floor using appropriate materials (quality plywood is my choice; stronger and lighter than OSB; if 4x8 sheets are unwieldy have the yard cut it to your rafter spacing typ. 16" or 24"). Then lay the floor as you advance into the attic, removing insulation as you go. Then add a few lights at the apex, then do your work that you need to do. Then put the insulation back over the new floor. Do the entire floor so next time you're up there you never have to worry about if there's a floor here.