With BCM (Bank Communication Management) grouping different bank accounts into one payment file not possible.

In App Manage Automatic Payments (App ID F0770)/Manage Payment Media (App ID F1868) Payment Batches and Payment Medium Files are generated at House Bank Acct ID level.

"Image/data in this KBA is from SAP internal systems, sample data, or demo systems. Any resemblance to real data is purely coincidental."

SAP S/4HANA Cloud 

Schedule payment in app Manage Automatic Payments

BCM is delivered as always integrated with BAM (Bank Account Management).

The Grouping Field House bank account ID (ID for Account Details) = HKTID is an integration requirement for the 2 solutions to cooperate.

In BAM the signatories (approvers) are kept at account level, hence it is necessary to ensure that batches are generated per House Bank Account.

System working as designed.

SAP Multi-Bank Connectivity

KBA , FI-AP-AP-B1 , Payment transfer (w/o DE, US) , Problem

SAP S/4HANA Cloud Public Edition all versions ; SAP S/4HANA Cloud all versions