the answer was, they will not support two services to the same address, if I try and order FTTP for my home it detects me as an existing customer and would treat it as a change to FTTP and not allow it as a second service to the home.
Sounds like odd (probably wrong) advice.
I've had 2 separate Talktalk services (I had an ADSL and an FTTC line) and know of a few others who have had 2 separate Talktalk FTTC lines at the same address.
All that was required was a unique email address for each account.
I was even able to use the same payment details for both accounts
If you order FTTP as a new customer with a different email address to your other Talktalk service then they should simply activate it as a new account.
It can't hurt to give it a try.
Just don't mention you already have a Talktalk account and see if they detect it during the ordering process.
If they detect an existing service and prevent the order then no harm done.
My guess is they won't pick up on it and will allow the FTTP order to be placed without affecting the FTTC.
They have recently been promoting 2nd lines for working from home so no idea why they would be blocking a 2nd service.
The Talktalk community forums also have a decent number of posts from users with 2nd Talktalk lines.
As you will have noticed during your enquires Talktalk don't have the best front line support.
Any question you ask that isn't on their script needs passed on to someone else or sometimes they just make up responses.
FTTP still isn't available on my exchange from Talktalk.
The FTTP team advised me my street must still be under BT's exclusivity period (they get 2 years exclusivity apparently) and when that runs out Talktalk will buy the license to sell it in my street.
I asked why could i order with Sky, EE, Zen, Cerberus, Giganet and about a dozen other FTTP ISP's and he said they must be sharing BT's license.
I corrected him, going in to detail on how GEA Cablelinks work and how there's no license or exclusivity period.
I don't believe Talktalk train their staff with that nonsense and believe the rep was making it up as he went.
I would take much of what they tell you with a pinch of salt.