The original intent, it would seem, is to flip one coin, multiple times.
That said, one of the reasons dice are tolerated as randomizers in OP is their efficiency. If the card text doesn't distinguish between the sequence of flips, i.e. you are just totaling up heads or tails from X number of flips, then rolling several dice simultaneously is fine.
If you have to check something after each flip, obviously, you roll one die at a time, so you can do your check.