There's a principle (will try to find the source later) that says that if one is in the midst of doing one mitzvah he is exempt from performing other mitzvot.
Do both mitzvot have to be of the same type (biblical or rabbinical)? Or does any mitzvah count towards this principle.
A common application of this happens daily: You have 5 minutes until sof zman kriat shema (a biblical obligation) and you're in the middle of reciting pesukey dizimra (a rabbinical mitzvah). Should you interrupt pesukey dizimra to say shema on time? Or does reciting the pesukey dizimra exempt you from saying shema on time?