Datamodelling 2 source DSO and one target DSO

Hi BI experts

we have 2 source DSOs:

The first source DSO has the following keys: 0FC_OPBEL and a Z-characteristic.

The second source DSO have four keys: 0FC_OPBEL, 0FC_OPUPW,  0FC_OPUPK and 0FC_OPUPZ.

The target DSO at this stage only have 0FC_OPBEL and the Z-characterstic as key fields.

The first source DSO at this stage is transformed and loaded firstly to the target DSO.

Now we need also the information of 0FC_OPUPK from the second source DSO to be transformed into the target DSO.

So, we recognized that we need this field as key field in the target DSO, otherwise we loose some information about this field.

The problem is, the first source DSO also have this field 0FC_OPUPK, but normally it is not filled up.

Do you know a solution or can you give us some hints how to model this challenge? Can we achieve to get all the information on one line even if 0FC_OPUPK becomes a key field of the target DSO.

Thanks

PC