Multiline for a synchronous interface

Friends,

Iam stuck with a webservice issue,

Scenario is File <->BPM <-> Webservice <-> BPM <-> Merge (File + WS) <-> Send

We have to make one sync call per order number (msg), right now the BPM is working for 1 call only. how can we mitigate this issue? iam not able to use multi mapping/ multiline container since its a synchronous call.

Thanks for all your help.

Regards,

Pavan