Certainly with the TomTom GO series that won't work, as there is only one connector. And equally certainly, my TT draws power from the computer; can do a full recharge if left connected. (The clue is that the little power light comes on.)
Speak as you find, once one is established as the right person, I don't have any difficulty in using their site.
What I do find intensely irritating is that the 'maps' function of me new all-singing-all-dancing motor costs...
£700+ 
and that's also Tomtom.
But I guess that's Toyota's pricing...