Since QuickBooks Self-Employed (QBSE) doesn't have registers and isn't a double-entry bookkeeping program, the ability to merge two QBSE accounts is currently unavailable.
Rest assured that I'll pass along your suggestion to our Product Developers. They're always looking for ideas to consider on how to improve QBSE. In the meantime, I recommend visiting our Blog site. This is where we share recent happenings and future developments, such as updates to newly added features.