I want to create a brass band with my friends at school. My friend is less experienced than me in playing the trumpet so I am the one who is most likely to do it. But my friend really wants to be part of my band. Am I allowed to have 2 trumpet players in my band or will that not work out?
I would say that having 2 or more of not only trumpets but any instrument depends on the style of music your playing. Having multiple instruments of the same type would add to the fullness of your arrangements. In addition to that, with the right level of experience and skill you can achieve some really nice harmonization.
Some examples of successful bands that include multiple horns are Chicago, Earth Wind and Fire, Kool & The Gang, Tower Of Power, and KC & The Sunshine Band (notice the bulk of that list are Funk oriented bands). Multiple Trumpets and Saxes are common in funk music.
This is purely my thoughts and there is no definitive right or wrong. It all boils down to the type of sound you want.
Why would the two trumpets be playing the same music? Obtain (or write) some arrangements with two independent trumpet parts.
What other instruments will you have? When dealing with ensembles including a rag-bag of available players at different levels, there will be great need of your music-arranging skills!