Hi,
From your scenario I understand that you require to serialize the idocs PO Creation + Change in a single interface (might be using multimapping) - Is this correct ?
If so, then in the receiver IDoc channel you need to tick the Queued option
I don't think you need any mechanism to populate the queue name - Its generated by ECC with SAP_ALE_XXXX
On the ECC side if the first idoc in the queue let's say creation ends up in error then the corresponding change idoc will fail. However the next pair of idoc will continue to process.
You can simulate above and check the behaviour yourselves.