This is a known limitation, and it does this on purpose. There's a technote on the issue, found here:
http://www-01.ibm.com/support/docview.wss?uid=swg21370331
In summary, the connection document (that these activities share) likely specifies a Permanent Notes View. Replication activities require an exclusive lock on a permanent view they are accessing, to prevent the corruption that can occur from external writes happening mid-replication. You either need to run the activities at separate times, or give each of the two activities their own connection document and view.