As per my LTE knowledge, I have never seen a case where data packets from multiple DRBs mapped to a single EPS bearer. In fact, each bearer establishes in network is unique in terms of QoS. And each EPS bearer is mapped to one DRB bearer at eNodeB. So I am thinking what would be the scenarios when it becomes to send data of different radio bearers to same EPS bearer.
Let's us assume one UE has connection with different PGWs but the UE is connected to same eNodeB. There might be a case, SGW is also different but unlikely since SGW is selected mainly based on UE location. However, there could be a chance due to overload at SGW, a new SGW has been selected for second bearer.
Now we have One UE, two different bearers, one eNodeB, two SGWs and two different PGWs due to different APNs.
In this case, how eNodeB would come to know whether it should combined the packets and send to same EPS bearer or it should not do like that. Even though we assume SGW is same for two different bearers and PGWs are different, it is logically in-correct to implement such case.
Would you like to share your use case so that I can think in that direction ?