Yes, you can use a different postbit in each style Peggy, however there is a slight trick to achieving this.
Because the postbit is determined at global level, it can't be changed by the user without a hack, alas - there is another way to do this. Read on...
1 - create your two styles
2 - on style 1 edit and make the postbit template the same as the postbit_legacy template
3 - on style 2 edit and make the postbit_legacy template the same as the postbit template
... and then because in each individual style the postbit and postbit_legacy templates are the same, regardless of which one you selected in the AdminCP as the default, the one will be postbit layout and the other postbit_legacy.