Just got myself a second (additional) US passport as I have to submit my primary passport for various visas and have to travel internationally at the same time.

Question on usage: Are there any norms on which passport to use when? For instance, if I visit the UK on my secondary passport next week, do I have to always use the secondary passport for UK visits in the future, or can I use the primary passport as well?

Any other tips to avoid inadvertantly getting into trouble by having two passports?

(Incidentally, having a second US passport is perfectly legal from the US point-of-view, as long as you have a documented need to a second passport).