As the double-backslash is also an indicator for a multi-value field, we first have to clarify if you have two composer fields, one filled with "AAA" and the other with "BBB": open the extended tags dialogue to see if there are 2 composer fields.
If you have 2 composer fields, then apply an action of the type "Merge duplicate fields" for COMPOSER first and use a unique separator like the underscore _ as separator.
As a result of this step you should have only 1 composer field with the contents
For this COMPOSER field you apply an action of the type "Import field (Guess value)":
Guessing pattern: %composer%_%lyricist%
You can join both actions in an action group and execute them in one go.