You must specify the creature you're helping, so your scenario works
The Help action in the PHB states (192),
You can lend your aid to another creature in the completion of a task. When you take the Help action, the creature you aid gains advantage on the next ability check it makes to perform the task you are helping with, provided that it makes the check before the start of
your next turn.
Alternatively, you can aid a friendly creature in attacking a creature within 5 feet of you. You feint, distract the target, or in some other way team up to make your ally’s attack more effective. If your ally attacks the target before your next turn, the first attack roll is made with advantage.
This section implies that you are giving your aid to another creature. In other words, the creature you're helping is the target of your help action. Therefore, you can choose to use your help action on the rogue. In this particular scenario, you must choose who you're helping: you can help the rogue, or the paladin, but to help with a condition like "whoever attacks the dragon next" you need to ready an action, which you can't do as a bonus action.
Given that you're a "master of tactics," such aid would be as simple as calling out a perceived, momentary weakness, or giving a word of strategic advice. Something like "The dragon has exposed his underbelly on the left!". The advantage only lasts for a single attack, and only for a single turn, so it has to be something brief.