DO IT!!!
Did they tell you why they want to pull a second report? It's kind of a catch 22. You already have a HP, so if you don't let them pull TU, you've gotten a HP for nothing. On the other hand, like you said, a second pull doesn't guarantee approval.
Can you call back and ask them why/what they're looking for?
Are your EX and TU reports similar? I wonder what they're hoping to find (or not find) on your TU. If the reports are similar and your EX doesn't qualify you, then your TU probably won't either. On the other hand, if they're looking for possible CAs (can't recall if you have any), and your TU is clean, maybe it will cinch the deal.
I'd probably see if I could get some more info first, to help you make a more informed decision. 
Credit reports are essentially identical, my TU is just always a few days later to update than EX (with balances etc). No baddies at all on my reports.
I actually just checked, my TU was unfrozen yesterday (forgot I had to unfreeze it since I was messing around with creditcards.com's credit card recommendation thing), and I already got a HP from AMEX out of it, since CreditKarma sent me a ping about it (but Citi's IdentityMonitor didn't yet... go figure) 
But it's frozen back up today again. Are they expecting me to let them pull TU /twice/ for one application?
Of course, the CSRs are all Peggy on that line, so answers are never clear. Not sure if I can get a US rep on the line somehow.
Congrats!!
Starting Score: 675