I recently ordered a welder that pulls up to 31 amps on 220. I have to add a 220 outlet to my garage. I have a lot of 10/2 w/ground wiring-solid copper. I was thinking of making a 60amp outlet. My question is can I run two parallel two pole 30amp breakers with two 10/2 wires going to the outlet? When I did my tankless water heater, I had to run two 8/2 lines w/40amp breakers(2). I didn’t know if I was allowed to do this for an outlet.
Nope, can't do that.
Your water heater has two separate heating circuits, each served by one breaker and one set of wires.
You cannot parallel smaller cables (for a typical house, just take it as a blanket ban - there are things that will not be found in a house that can be parallel wired, but the parallel wiring requires devices listed for that, and involves minimum sizes that are far larger than anything but your main service cable would have.)
One breaker, sized as the welder manufacturer specifies, connected to one set of wires sized to be protected by that breaker.
Incidentally, it's been 240 in the US for about 50 years now.
You'll be fine running 10/2 wire that you already have (however if it is NM type cable, it cannot go outside or in wet locations.)
Here's why: Welder rules are WEIRD. NEC Article 630. We're not in Kansas anymore!
Specifically, the welder rules give very favorable consideration to the duty cycle rating of the welder, and are perfectly OK with the wires in the walls being somewhat overloaded during that short duty cycle. Since they will cool off the rest of the time.
It's one of the rare cases where you can put a larger breaker than 30A on #10 wire. See the exception list in NEC 240.4(D) (which otherwise limits #10 to 30A).
So my advice is to wire it as a 30A breaker to #10 wire to a NEMA 6-30 receptacle (NEVER use a NEMA 10-30!) Then, see how it goes. My guess is, it'll never trip. But if it does trip, then crack open Article 630 and see how much you're allowed to enlarge that breaker.
The type of paralleling you are thinking of is never allowed in residential. Aside from the 5 paralleling rules you'd find on the Internet, there's a hidden rule that says the source equipment must be UL Listed for paralleling. And that brings in a raft of requirements which drives prices over the moon and makes it impracticable. Not least, UL requires every conductor have a separate breaker or fuse.