I have close Spanish friends who use "vosotros" when addressing me and my husband. As I speak Mexican Spanish, I never learned this form of "you-all" and use "ustedes" instead. Is this impolite? Do my Spanish friends think I am trying to keep a distance between us? And is it wrong for me to address children as "ustedes"? Must I go back to school and learn the vosotros conjugations?
The issue of second person pronoun usage is something that often comes up with speakers across the Spanish speaking world. Even in countries where the only three are tú/Vd/Vds, the customs of using each one will be different, and adding in vos, sumercé and vosotros just adds more options: the underlying "problem" is the same.
The reality is, however, that most Spanish speakers are somewhat accostumbed to this. If I hear a Costa Rican call me usted, I don't necessarily assume much by it, just as when they hear me call them tú they tend to understand by my very different accent that I'm not used to the same formality distinctions as they are. As a foreigner, you will be given even more latitude. There are many other elements of speech — word choice, single pronoun usage, etc— that can help a Spanish speaker understand whether you are intending to be offensive/polite/friendly/etc.
Specifically in your example, if you regularly use tú with your friends individually, but then use ustedes for both, there will be no confusion: it's clear that you're using an ustedes-only dialect, and by virtue of using tú that you intend to be friendly/familiar with them.
You won't be expected to modify your dialect, and that's common across the Spanish-speaking world, although if you live for an extended period of time somewhere with a different system it might not be a bad idea to try to pick it up.