Since it’s DC power it’s likely ok for the most part, but if the thrusters get turned on suddenly or whatnot there might be power spikes that could corrupt the occasional communication. If it’s not a huge issue to have a slightly damaged video frame or missing mavlink message from time to time then the lack of shielding is likely ok in your case.
Two tethers is possible but not suggested, because it significantly increases the risk of a snag. It also requires attaching them together at sever points along the length, which makes it harder to wind them up and means you’ll likely end up with extra bits of junk stuck between the two cables.
The OTPS cable is indeed quite expensive, so you might want to look at alternatives from some of the Chinese manufacturers who can make you a custom cable with the actual length and characteristics you want. Alternatively one of the bluerobotics engineers might be able to give you a current specification for the existing Fathom tether. I’ve also just realised that since the fathom has 4 twisted pairs, if you’re only using one or two of them for comms you’d likely be fine if you use one pair for power and one pair for ground, since two 26AWG wires > one 23AWG wire.