Electrical – 30 ampTankless Water heater

Interesting. In Canada the cable you need exists (2 conductor 12 or 10 AWG, red and black jackets, with bare ground), is commonly used to wire baseboard heaters at 240V and 20A, and can be found at any HomeDepot. The cable is even made in USA! See here for 12 AWG: https://www.homedepot.ca/en/home/p.electrical-cable--copper-electrical-wire-gauge-122---romex-simpull-nmd90-122-red---10m.1000108220.html; and here for 10AWG:https://www.homedepot.ca/en/home/p.electrical-cable--copper-electrical-wire-gauge-102---romex-simpull-nmd90-102-red---75m.1000108211.html

If you can't find it in the USA, then see Harper's taping solution in another answer.

Illegal, and impossible at any sane cost

Paralleling is, for all practical purposes of interest to you, illegal in mains wiring. Even when it is done owing to ampacity needs, it uses special equipment on the line side which renders it useless for anything like this.

The only way to inter-tie two supplies is with some sort of airgapping which assures energy flow is one way always, and provides isolation. Think of it as "magic diodes that work on AC". M-G sets with overrunning clutches, or railway-style substations where you feed an ungrounded transformer then rectify to DC and run your machine off the DC bus. Any of these would be prohibitively expensive, of course.

The path of least resistance (heh) is to have the conversation with the power company. You need to do this anyway, as you cannot simply slap a 175A load on their line without permission. Their supply lines and equipment are simply not factored for a load that large! Most houses take 1kw (4A) on average, and they absolutely rely on residential loads being intermittent.

Expect a lot of things to come out of this conversation, such as 480V - you could run your load off three #6 wires instead of two #0000's..

Also did you remember the 125% derate?