Hello. You have not informed us which plots this question refers to, but we can say that a parallel plot creates tension, when the two plots involved present unpredictability in relation to the events that compose it, without cutting the sharing of elements between them , where what happens in one plot can totally influence the other, but the reader cannot understand how it will happen and is anxious about the tension that is established in the narratives.
In other words, the tension in a parallel plot is created by the unpredictable interaction between the two plots.