either... or ?

Mal ... mal... would normally be:

Sometimes crossing the path of the lines, sometimes merging with them, these shapes are reminiscent of...

But if you say ... und sich dann wieder irgendwie in die Linien einfügen...

then maybe:

First crossing the path of the lines, then merging with them, these shapes are reminiscent of...

Or even:

Crossing the path of the lines, then merging with them, these shapes are reminiscent of...

Phrases

3a. now (one thing), now (another): (used to introduce antithetical clauses, phrases, or words) at one moment one thing, at the next, another; alternately one thing and another

[OED, now, adv., conj., etc.]