Data Integrity - Finance

We are going to split our systems from another company and are questioning how data currently in our system will need to be handled in reference to the other company. We are planning on pulling in only data related to our companies, however, there could be journal entries or AP records that share data with both companies. Do we need to bring in all records to balance the JEs and APs or is it possible to leave that data out without causing issues? What are the risks of not bringing in data for entries in our system that are no longer part of