I think this would be a simple change. When placing a pipe from the mainhand, check if there's a pipe in the offhand - if yes, attempt to place the offhand pipe after placing the first pipe.

It would basically call the onItemUse (or whatever the equivalent code is nowadays) for the offhand pipe after placing the mainhand pipe. Therefore everything except the "check offhand for a pipe" would use existing code. If it's as simple as I hope it is, it would be just like placing one pipe then switching to another pipe and attempting to place it.

I want to double fist my pipes, please 😩

You also can't place 2 blocks in one spot, but you can with MI pipes and cables :)

I'm holding out hope - one day..

I think that would be super cursed. It's not like you can place 2x1 block columns like that...

Not gonna do it but I like the creative thought. :P