Hmm, I think the answer will be different for everyone, there’s nothing stopping you from doing that so you can if you want
Personally I would probably wait to ensure I won’t get confused and have to be working to edit/fix two patterns at the same time, and that’s not including the potential questions that may arise between the two sets of testers for the patterns, but if one is ending/the testers are almost done I would probably open another one
As for announcing posts too often, it should be good, as long as it’s not on the verge of being spammy you should be fine, I know self-promo posts less than a week apart verges on being spammy, since it’s just testing it should be okay, if you’re worried you can always consult the community guidelines or if the patterns you want tested are similar you can post the two in the same tester call saying for testers to apply to which they want to test if that makes sense 