okay, say you have a team consisting of R Hyperion, V Taskmaster and E Transporter Spider-man.

So, Taskmaster and Spider-man wildcard to Sinister Syndicate and Taskmaster is hidden on some stealth. Six spaces from Taskmaster is let's say E Shadow Cat.

Now, if Spider-man is transporter attacking, can he borrow Taskmaster's attack mid-move, and if he does, does it become a 12 or get the -2 attack and is only a 10?

He can so long as he attacks while adjacent to Taskmaster. It becomes a 10. Replace with the 12, modify to the 10.

ZZZ is right. Since borrowing the attack isn't an action of any kind, just an automatic replacement. So it is totally possible to swipe that attack value and modify it down.

A couple things here.

First, the action as you describe it is perfectly legal. However, it is important to remember that during HSS option 1 and transporter move and attacks, the attack occurs during the movement. Meaning, if Spider-man was carrying Taskmaster, Taskmaster would be off the board during the attack and would not be available to lend his AV.

Second, when stuff like this is occuring at the same time, the mantra is "replace then modfiy". So, first Spidey will borrow the 12 attack, then it will be modified by -2 for a total of 10. The key here being "occuring at the same time". If instead, V Taskmaster used his Perplex on Spidey's AV, then tried to share the AV, the Perplex would be lost and completely replaced by Taskmaster's AV since it occured before the attack (and thus the sharing of AVs).

I hope that helps.