I do see the value in doing this. Not necessarily multiple pairs in parallel, but with some overlap between them.
But first of all, if this is your first time wearing any HAs, I would only start out with 1 pair and let your brain get adjusted to wearing HAs in general in the first place. This may take a few weeks.
Then if you want, you can get a second pair half way through your first pair’s trial period, while you still have the first pair. But I wouldn’t alternate wearing them daily. That would just be too confusing. I would wear one pair for a solid week in all listening environments first, get an impression of what I think of them, then switch to the other pair the following week and wear those for at least a full week. Do this a couple of rounds, and you should have a solid impression of which ones you like better.
I would also save and try to trial the pair that I think I’d most likely keep last. Of course you wouldn’t know which one you’ll like more for sure until you try them. But at least if you hedge your bet this way, and it turns out that your guess is right and you like the last pair you trial more, then you will have made the most of the trial periods for both pairs if you overlap them like that. Otherwise if you like the first pair better, you’d have to return the second pair sooner than its full trial period if you decide to keep the first pair because its trial period expires before second pair’s trial period.
But for Costco with 180 days trial period, it probably doesn’t matter because you’ll have more than enough time to make up your mind with both pairs anyway.