"These two also come by now and then."
My assumption from the use of "mal" in this lesson is that, when you talk of someone going somewhere/arriving somewhere, and timing is implied (i.e. these two come by now and then), "mal" has to be inserted after the clause that indicates time (i.e. "ab und zu mal," which would directly translate to "now and then times"). Is that right?
"Mal" is often used as an emphatical marker without any particular meaning. German is full of such that have no direct translation into English but help to express emotions and nuances.