Agree the source of truth
Decide which system owns each important record and which direction updates should flow. Define how duplicate records and conflicting changes will be handled.
Design for exceptions
Clarify validation rules, retries and how users will be informed when information cannot be transferred. A visible exception process is as important as the normal workflow.
Test real business scenarios
Test typical records, incomplete information, timeouts and repeat submissions. Include business users in acceptance testing so that the connection supports the actual process.