Fear the Walking Dead: Dead in the Water" is a six part webisode/prequel of Season 6, and provides exposition into the importance of "THE" key, and the submarine "USS Pennsylvania" featured in the season.
The protocols for arming and launching a nuclear missile from a ballistic missile submarine shown here are somewhat accurate, but not overly so. First off the keys carried by the Commanding Officer (CO), Executive Officer (XO) and Chief Weapons Officer (CWO) are not the keys that arm the missile launch sequence, those two keys are stored in a safe locked with a combination lock, no one on the boat knows the combination to this lock. The keys that these officers carry are to a safe that contains an envelope that has a verification code printed on it, it takes two keys to open said safe. When a fire order is received it includes a verification code that must match the printed code in the safe, these safes are guarded at all times by two armed guards who are to use deadly force against anyone who attempts to access either safe or the fire controls without following proper procedure. So when the fire order comes in the CO, XO and CWO must all do an initial verification by reading the orders to ensure that they are formatted correctly, there is a very specific format and encoding sequence used for these orders, the details of which are obviously classified. Once all three officers independently verify and agree that the order is authentic they then proceed to open the safe with the envelope that has the final verification code printed in it, once the safe is opened all three officers must independently verify that the code from the safe envelope matches the code in the fire order, this envelope also has a cipher key (the safe combo is encrypted in the verification codes) which is used to obtain the combination for the safe that the fire control keys are in. Once the last safe is open the fire control keys are usually taken by the CO and CWO, but any two of them may do it, and the armed guards in the room will use deadly force without warning if someone attempts to skip any step of this process. As shown in the episode the slots for the fire control keys are placed on opposite sides of the room so that no one man may use both keys at once. Something else the episode got wrong was showing the CO turn his key and then order the CWO to turn his key as well, part of the fail safes in place is that no officer can be ordered to help arm the missiles, everyone must agree that the orders are valid and the situation calls for such use of force. Another fail safe is that both keys must be turned at the exact same time, one key cannot be turned and then the other after, that would defeat the purpose of the two-man rule. If both keys are not turned at the same time, within half a second of each other, the system will not arm.