Generally our habits formed through our personality, which express our different psychological elements, condition us into a sleepy, somnambulist state, consciously-speaking. This is why it is good, when possibly, to try to do things separately, when practical. This is not always the case in our busy lives, particularly in modern Western society.
However, what is most important is this: when we perform an activity, we give to it as much direction and attention with our consciousness as possible. This means that if we can reduce certain habits where we like to do many things at once, albeit superficially, into singular actions with much more dedication and focus, we will typically find the product to be much more profound and rewarding. Knowing when to single out activities is a personal thing, depending on your obligations and needs. But typically, if we are listening to a lecture, we should sit down, relax, even close our eyes and meditate on what we are receiving, because this is a superior type of knowledge being transmitted, so we must therefore show respect by making our minds receptive and integral.
When it comes to mantras, it is best to practice when isolated and in solitude, without distractions or activities of any kind. However, it is also beneficial to use mantra in daily life, pronouncing the sacred words mentally. This takes much more skill and tenacity, because it is challenging, especially in the beginning, to learn how to negotiate the internal with the external world in a spiritually-cognizant way. With enough discipline, we will know how to mantralize when interacting with people, when eating (such as with the mantra "KRIM"), when driving the car, when giving a lecture. It is challenging, but can be accomplished in the Yogi who shows indefatigable will and endurance.
Yet since most practitioners are not at this stage, we typically advise students to not multitask, but try to divide more time in the day towards singular activities, thereby completing them with much more focus and awareness. Yet as one develops spiritually, such obligations become easier to fulfill, and with maturity in one's consciousness, one can move on to greater and more challenging things.
Joyful in hope, suffering in tribulation, be thou constant in thy prayer.
Benedictis, qui venit in nomine Domini. Osanna in excelsis.
"Blessed is he who comes in the name of the Lord. Hosanna in the highest!"