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Bit hard if it's a 2hr climb and both riders are similarly paced/similarly fooked . There'd actually be more advantage to them dropping back then passing then the other riding passing repeatedly vs. if they climbed side by side.
Also, if the riders do have a chit chat side by side, they're both 'guilty' so hardly likely to both dob themselves in so it would have to be someone else catching them out.
I rode side by side with a few people and chatted. However, I think it is against the rules and would have been much more cautious about doing it if I'd been anywhere near the front of the race.
The manual deliberately doesn't define drafting but says refer to your local TT rules. And as CTT says drop back if passed, side by side wouldn't be allowed.