Sync
Use the same email on every phone. Invoices, clients, and stock follow that email.
Create admin account
Choose the email and password you will type on every phone. After this, the shop syncs by itself.
Or use Google or X. The shop still follows that sign-in.
Sync with a shop file
Use this when the sub-user phone has no live sync. Send the file on WhatsApp as a document. The other phone taps Open shop file. Work already on that phone is kept.