It depends upon the type of plan you are going for, but you would certainly qualify for some form of multi-line discount.
The way this currently works is you would get a £12 discount off full-priced SIM Only plans or Flex Pay airtime plans if taking a phone on a device finance agreement (or exclusive additional line pricing on either).
Or if taking an older style 24m Phone plan with a combined cost you would get a 10% discount on the plan.
This will likely depend on credit checks/status of your account etc as to which kind of plan you can take at the time.
You will be able to get an idea of what deals are available through your online account at any time and this will give you a rough idea of what it'll be.
Important to note a lot can change depending on when you are doing this, if you are waiting until next year any advice given now will most likely be out of date by then.
