The Custom Report section of Blackbaud supports only one database export per report. Reports that use multiple databases can still be run within the Crystal Reports application but cannot be set up to run in the Custom Reports section.

An alternative would be to use an ODBC connection to the database. Please review KB BB126231 on how to use the RODBA module to work with Crystal Reports.
Please contact the appropriate software vendor or IT professional for assistance with this process or issue, which is beyond Blackbaud's scope of support.