Solution
These limitations are inherent to NI-XNET. When using project sessions, it may not be possible to call other DBCs or clusters. To address this, it is recommended to either use the Create Session VIs or combine the CAN frames into a single DBC, allowing for the use of multiple project sessions.
It is particularly recommended to use the Create Session VIs, especially if updated DBCs are expected in the future. This approach avoids the need to repeatedly merge two DBCs into one just to use the project session option.