Baseline grid is a document property not a page property. So there is only one main baseline grid for a single document. If you need exceptions you can use a baseline grid that is tied to a text frame.
Baseline grid is a document property so you can only set it per document. You can set alternative grid for text box or, for a large publication, you can create multiple indesign publications and combine them in on book.
Build two separate masters; not based on each other. On each master page you build text frames. On each text frame go to Text Frame Options > Baseline Options > Use Custom Baseline Grid. Your leading of master A text frames can be different from master B text frames. You edit your BodyText paragraph style to Align All Lines to Baseline Grid. You can even link a text frame supplied from A to a text frame supplied by B; have text flowing through them; and the leading will change automatically. So it IS possible to have more than one baseline grid system in a document. You essentially ignore the one setup in preferences.