Are store cashiers (Target) trained to start conversations/flirt with customers?

No, cashiers are not trained to flirt with customers. Customer service always takes priority in retail jobs, which means that being nice to all customers is a job requirement. I used to be a cashier and a number of guys got the impression that I was flirting with them when in reality I was just being friendly and doing my job. However, it's possible these girls were flirting with you and your friend. There's nothing wrong with asking them out; just know that it could be really awkward if they say no and you may wish to start shopping elsewhere.