Hi Anton,
I don't think there is a standard approach defined in PC for such cases.
Nevertheless, it could be a possibility to define a programmed rule in one of the two back-end system and in that programmed rule logic (abap OO logic), code a call to the second system to read the needed data.
Then, from a GRC point of view, you remain standard: one programmed rule defined with the data source linked to one system (which will read the second one in it's coded logic).
Doc of programmed rule (if needed):
Kind regards,
Pascal