Sort order on partition, but not within partition

Hello,

We need to collect data from a table with multiple partitions but in a very specific order. Actually the needed data is spread over 2 of about 90 partitions. In the first of the 2 partitions (in fact in our case the p_null partition: partition key is NULL, containing only a few thousand records), essential header data is loaded (to be precise: this header data contains several headers, describing the content of the data contained in all the other partitions). In the second millions of related data records exist. 

We now want to retrieve first the header record from the first partition (filtered on a documentnumber) and then all records from the second partition. The order in which the data is retrieved from the second partition is irrelevant.  If it is retrieved in the wrong order, so header data somewhere in between normal data, a dependent process which relies on this right order will fail.