You need to keep in mind that this sort of structure has inherent risks:
- Most obviously, it’s hard to generate suspense in the past setting. If it looks like the hero’s going to get killed, it’ll be pretty obvious that she makes it out alive, because she’s alive in the later storyline. On the other hand, you can create another kind of suspense based on our knowledge of what’s going to happen: “Uh-oh, this guy is acting nice, but I know he’s going to turn out to be bad! When’s the other shoe going to drop?”
- You have to motivate the jumps. One book was simply organized around “Present day 1, past day 1, present day 2, past day 2, etc.” Ideally, you should always have something happening in the present to cause the jump to the past, then have something in the past cause the jump back to the present. She looks at her scar, then flashes back to the day she got it. At the end of that sequence, someone looks at the wound and says “Do you think it’ll leave a scar?”, then we cut back to the present. It can’t always be that neat, but look for ways to subtly help us make the leap.
We have to know when “now” is. You can’t have two nows. You need a now and a then. We have to identify with one timeline more than the other. We need to set our feet down in the present storyline and leave them there while we crane our neck over into the past storyline for a looky-loo (even if we spend most of our time there). One timeline should provide perspective on the other timeline. Both storylines should be from the same POV: the present POV. Thus one is present-tense and the other is past-tense. (Or it also works to have them both be past tense.)
Another reason to do this: It’s very easy for your reader to forget whether they’re in the present or past if you’re spending big chunks in each timeline. This way readers can just check the tense, but they probably won’t need to, because you’ll be subtly reminding with each sentence.