Mech cockpits generally aren't, though. Have you ever really studied the art? Some of them are so small I can barely picture one person fitting inside, let alone two.
The Assassin is another one too. In lore the mech is so cramped in the cockpit, that MechWarriors that are assigned to them often develop back problems.
"The Assassin was not flawless however,... it has the dubious distinction of having one of the most cramped cockpits of any Inner Sphere 'Mech. Dozens of attempts by Maltex over hundreds of years to overhaul the cockpit cooling system to create more elbow-room ended in failure, not only limiting Assassin pilots' time in-cockpit to half of that of other 'Mechs, but causing many to develop a lifetime of back problems." - Sarna.net
Overall though, I get the feeling that our head mech technician, Yang Virtanen, seems like a rather skilled tech capable of a lot ingenuity and creative work-arounds. I could see where he could squeeze enough room into a Spider cockpit to accommodate another person (although I doubt they will be comfortable...or tall lol). Think of the additional room for a VIP as sort of "Gurney Bubble", but for a Spider.
Basically, I can see where a person can take that bit of lore as law, and I can see where it can be adapted to fit a narrative. It just depends on how they want to take it. It's their game, thei world, they should play it how they like

.