It's an unusual design, I'm not sure if DbAdapter is designed to poll in a mid-process receive... I guess you can use a secondary BPEL process to overcome...
Please refer the following sample for your usecase.
Though Please be aware that one instance of BPEL process will correlate with only one message if you have one receive activity. Not all the messages tht DBAdapter polls.
bpel-305-InboundCorrelationShows how to perform message correlation within BPEL
Also Refer the following in Developer guide.
If you want to keep polling and receiving the messages in the same BPEL process you might have to put the receive activity in a while loop construct. But as a design this is not a good design.
Can you elaborate your requirement to understand better and suggest better solution.