How do you handle fast players in a casual setting that skip priority?
By the rules, you allow opponents to backup and counter the first spell.



MTG is not a real-time game, it's a turn-based game. In other words, it's not a game of chicken.

MTG would be extremely tedious if you had to explicitly pass priority every time. That why people use shortcuts. If you say "I cast Foo and Bar", you are actually proposing the following a shortcut:


I cast Foo
I pass
You pass
I cast Bar
I pass
You pass


The other player(s) may accept the shortcut entirely, or they may accept the shortcut only until a point of their choosing. Accepting part of the shortcut could be done, for example, by saying "In response to Foo, I cast Baz."

If the shortcut is accepted, the person who proposed it must abide by it. (He must cast Foo and Bar.)

If the shortcut is accepted to a point, the person who proposed it must abide by it up to that point. (He must cast Foo.) He can deviate as he wishes past that point. (He need not cast Bar, though he obviously can if he wants to do so.)

So the answer, by the rules, is the last option you presented: You allow opponents to backup and counter the first spell.

If you want to be sure Foo isn't going to be countered before announcing you're casting Bar, you have to make sure noone wants to do anything in response to Foo.