I would water the spot really well, wait a day or two, until it sort of dries out, but not completely, then dig it up in a big circle around both of them and then very carefully start removing the soil until you can find the outlines of each individual plant. And from the top, carefully start to pull them apart from one another. Then replant each of them.

Last time I did something similar to this I dug them both up and gently hosed them dirt clump down while carefully untangling. I've seen people do it in a five gallon bucket too just by dunking over and over and gently pulling

I agree with @Adverse_Reaction@anarchist.nexus, but you could substitute a shovel for a garden fork for loosening the soil around the roots. Sometimes a sturdy four tine fork makes things a little easier digging out plants of this size when you're trying to tease them apart but maintain your soil aggregates (within reason).

If you're in new england this weekend is a good time to split them - we're due for some major rain events next week and that will give them a break from heat stress while also helping to water them in, which will both help mitigate the transplant shock.

All the higher effort options posted here are very good and you should do them. But if you can't muster it for time or care, you could probably just water them really well, wait till the next day, and then gently pull the New England Aster up and put it somewhere else. Either way, good luck, it's going to work out regardless.