Currently only a single custom price book (CPB) can be assigned to a given payer account.
This is currently a limitation of CPBs and only a single price book can be assigned to a billing account.
Hence, there can be multiple CPBs, but only one per each Billing Block / Account.
By design, CloudHealth applies one partner billing rule to All billing accounts without choice. A workaround is to Zero out the line items for unnecessary partner billing rules by using CPBs or the rules for a specific account can be consolidated into one CPB per billing account.