Berachot 2.5 - The general principal of "One who is involved in a Misvah is exempt from another

"Ha'psek b'misva patur mi'misvah" The general principal of "One who is involved in a Misvah is exempt from another misvah that he now does not have to do at the same time. Just as you can't dance in two weddings at the same time, if you are engaged in a good deed, you are not obliged to stop what you are doing and do another one.