The deal gets announced, the practice groups are mapped, the compensation model is agreed, and somewhere in the second month somebody asks how the document systems are going to work.
By then the answer is usually "both, for now," and "for now" has a way of lasting three years.
What actually has to converge, and in what order
Not everything needs to merge, and trying to do it all at once is how integrations stall. The sequence that works runs roughly like this.
Identity first. One directory, one set of credentials, one way to remove somebody. Until this exists, every other system has two access lists and no reliable way to offboard a departure across both.
Email and calendar next, because these are what the outside world experiences and what makes the firm feel like one firm internally.
Time and billing after that, because it determines when you can report on the combined business, and because running two billing systems past one full cycle is where reconciliation costs compound.
Documents last, and carefully. This is the one with the confidentiality exposure, and rushing it is how a matter ends up visible to somebody who should not see it.
Everything else on its renewal date. Practice management, knowledge systems, marketing tools. Let the contract cycle drive the sequence rather than fighting it.
The obligations problem
Here is the part that is genuinely different in professional services, and the part most integration plans skip.
The two firms have different confidentiality commitments. Different client engagement terms. Different data retention promises. Possibly different regulatory obligations, if the practices span jurisdictions. And almost certainly different conflict checking practices.
When you merge the document systems, you are not just moving files. You are placing information governed by one set of promises into an environment governed by another. If firm A promised a client that their material would be accessible only to the engagement team, and firm B's document system is open by default across the partnership, the merge has quietly broken a commitment nobody reviewed.
That review is a short piece of work and it has to happen before migration, not after. It is also the piece that a technology integrator will not do for you, because it is not a technology question.
Ethical walls have to survive the migration
Related and equally easy to lose: any matter-level restrictions in the source system need to be mapped and reproduced in the destination before content moves. Permissions do not migrate reliably by default, and the failure mode is silent.
The test is simple and worth insisting on. Pick three restricted matters. After migration, have somebody who should not have access try to open them. Do that before the bulk move, on a sample, not after.
The mundane thing that causes the most pain
Departing staff. In the year after a merger, turnover is elevated and offboarding runs across two of everything. Every system that was not consolidated is a place a former employee retains access.
If you do nothing else structural in the first ninety days, get to one identity system and one offboarding checklist that covers both estates.