I'm struggling to get both of my cats to use their new litter box

I have two cats, one is 6 years old and the other a little over 1. I recently got them a new automatic litter box (don't worry, it's one of the safe ones) and have been trying to get them to transition over. For now, I just put the new litter box right next to their old one (which is also a different style of auto litter box). I put a bit of dirty litter in the new box and even a couple of pieces of poop as a cue so they would know what to do.

At this point my older cat pees in the new box but hasn't pooped in it yet. My younger cat has only peed in it once or twice but will not stop laying in it and just hanging out in the litter. I don't want to chastise her bc I want to encourage her using the new box but I obviously don't want her laying in the litter. It has been around two days and I feel like all progress has stalled. I don't know if it's because the design of the litter box is drastically different or that I have to use a different litter type from what they are used to but I'm not sure where to go from here.

Should I just throw the old litter box away entirely so they only have the one option? Part of me is just worried that they will start going somewhere outside the litter box.

tl;dr: one cat pees in the new litter box but hasn't pooped yet. Other cat just want to hang out and lay down in the new litter box. Should I just get rid of the old box so they only have one option?