How to copy values from one CDO field to another CDO field ?

Summary:

Use-case:
We store CRM leads in a CDO, say CDO A.
We store data from another integration into another CDO, called CDO B.

In order to sync the info from CDO B to CRM, we need to check if it is an existing lead.
If yes, we need to copy the lead ID from CDO A to CDO B.

Has anyone worked on a similar scenario ? How to handle this use-case ?
Need a solution other than exports/imports via APIs, using an intermittent contact field (as we have already surpassed the contact field limitation).

Is there any other way to copy values form one CDO to another CDO ?