[Toni]

Me contesto yo mismo con una respuesta de un foro de 'Borland' por el señor Jeff Overcash:

This means the transaction is started before you called ApplyUpdates.
DataSnap only commits the transaction if it starts it. You should
never touch the transaction for a DataSnap driven IBX query. The
transaction's AutoStopAction should always be set to saCommit and
PacketRecords needs to stay -1. This causes the transaction to be
closed after reading the data so it is in the right state when the CDS
tries to Apply the updates.

Y en el que tambien comentan el uso de varias transacciones sin problemas. En el caso de utilizar CDS+DSP+QRY no hay que utilizar nunca las llamadas de commit y rollback directamente al TIBTransacction.