You will need the Full Account permission, with no campus limit, organization limit or campus override, to archive an organization.
Going back to one organization means archiving the second of your two active organizations. That archive does everything a normal archive does, and then more, because your account stops having more than one organization. Multi-organization turns off, staff limits are rewritten, and some settings are trimmed back to one.
This article covers that second half. How do I archive an organization? covers the archive itself: the eight checks, the choices for people and records, and what happens to the archived organization.
Warning: Going back to one organization deletes every organization limit on every user and role. Anyone left with no campus in their limit loses the limit completely and can reach everything in your account. Check your staff and roles before you archive.
Before you archive
Work through these steps before you open the archive window:
List the staff limited to the organization you are archiving. Decide for each person whether they should keep access to everything or lose staff access. The archive window asks you to choose for each of them.
Check who holds each role that is limited by organization. Roles cannot be removed in the archive window, so a role left with no campus in its limit reaches everything, and so does everyone who holds it. Change the role's limit, or who holds it, before you archive.
Check Campus Attended. The archive window shows how many people will have it cleared (see below).
Settle the archive's own checks and choices. See How do I archive an organization?
What happens to staff access
This is the part most people do not expect, and the reason to plan ahead.
Every organization limit for every user and role is deleted. Campus limits at the archived organization are deleted too.
Campus limits at the remaining organization stay. A user limited to one of its campuses is still limited to that campus.
A user or role left with no campus in their limit loses the limit and reaches everything. This widens their access, and it happens unless you choose otherwise.
The archive window lists the users whose limit names the organization you are archiving. Nothing is chosen in advance. For each user, or for everyone at once, choose one of these:
Keep access - the limit is cleared, and the user can reach everything in the account.
Lose staff access - the user's staff access is removed, and they are signed out after the archive.
The archive is refused until every listed user has a choice.
Example: Pat is limited to Riverside Academy. When the Academy is archived and Riverside Community is the only organization left, Pat's organization limit is deleted. With no campus left in the limit, Pat would reach everything. The archive window lists Pat, and you choose whether Pat keeps access to everything or loses staff access. Sam, who is limited to Riverside Community's North campus, keeps that campus limit and is not listed.
What happens to records
Every record held by the remaining organization loses its organization and returns to All.
Records held only by the archived organization keep it, and stay read-only.
Records with no campus go back to the account's time zone. Campuses keep their own time zone, or keep inheriting the remaining organization's.
What happens to settings and sharing
Dashboard, form and chat sharing that names only the archived organization is removed.
The account keeps one default designation, one giving page and one merchant account, all from the remaining organization.
What happens to people
Some people belong to the whole remaining organization rather than to one of its campuses, and also list a campus as their Campus Attended. Their Campus Attended is cleared. The archive window shows how many people this affects before you archive:
[count] people belong to all of [organization], not one of its campuses, and list a campus as their Campus Attended. It will be cleared.
What happens to giving and books
Going back to one organization does not merge your books or your giving.
Giving and money stay split by organization while the archived organization still owns a gift, batch, recurring gift, pledge or journal entry.
The books stay split while the archived organization still owns a journal entry.
The archived organization's add-on seats stay listed, but they hold nothing.
How do books work with more than one organization? explains how split books work.
Adding and archiving are not opposites
Adding a second organization runs a one-time cutover that cannot be undone. Archiving back to one organization does not reverse it. It is a separate change with its own effects, the ones listed above, and some of them, such as deleted staff limits and cleared Campus Attended values, do not come back.
If you add a second organization again later, the cutover runs again. How do I add a second organization? explains what it does.