Re: New beek game plan
If they die of mites, it's normally during the winter and at that time you probably cannot get a queen.
Carniolans are great bees and your packages should grow explosively, long as there is nectar coming in, or you feed them. So to have a good shot at a nice honey crop this year, keep the carniolan queens.
But if the ultimate plan is to requeen with (supposed) mite resistant bees, plan that for early fall.
I suspect your plan is really to see how the existing bees handle mites on their own. To know that you'll need to test the bees for mite levels. However there are a number of traps for new players when doing this, and it is easy to be mislead by interpretation of results. My opinion, would be just go ahead & requeen at least one of them, with a resistant queen, you could leave the other hive as is, if you think they are tolerating mites, as an experiment.
Course you have many options, those are just my thoughts, there will be other opinions equally useful.
44 years, been commercial, outfits up to 4000 hives, now 120 hives and 200 nucs as a hobby, selling bees. T (mostly).