It sounds like they've sold you another SIM (the promotion is unclear as EE do not offer a SIM that lines up with £16pm) plus a 24 month 50GB plan for £14.
The £14 50GB plan is only for additional lines, so by selling it this way they've been able to give you the £14 offer, as opposed to what you should have usually been entitled as a new customer which is £17 for 25GB or £19 for 100GB.
They shouldn't be selling this way. It is misleading for the customer if not explained correctly, which sounds like the case here, but I'm also pretty sure it will violate their employee codes of conduct (and potentially OFCOM rules) to sell a product which they know the customer will ring up and cancel.
Without knowing the details of the first SIM they gave you (the tariff and the minimum term) it is impossible for us to say if this is even possible.
If they've sold you the first SIM as a 1-month rolling contract (which is what I think they would have done if they're advising to cancel it) then theoretically you would have to pay for 44/45 days of service on the unnecessary SIM as the standard cancellation policy is 30 days notice which they have advised you to give on Day 15.
I however can't see a 1-month rolling SIM which lines up at that difference in price, so I am really not sure what they've done with the first tariff...
The good news (relatively good news i suppose) is that I've just looked and the £14 50GB additional line SIM looks to just be a £14 tariff, so by cancelling the other SIM this should not jump in price as a result of a Multi-line discount falling off.
But they really shouldn't have done it this way, to try and give you an offer you're not entitled to, and to inflate their own sales numbers by selling 2 SIMs as opposed to one.
