@TinH Hi Tin, 

OneMesh requires a tp-link router in your network , and WiFi client switch Wi-Fi fast between router and powerline devices. 

2 Powerline KIT could not build a oneMesh network. But you could set up same SSID and Wi-Fi password for 2 powerline devices, and Wi-Fi client could roam between powerline units, but it has short cut-off when roam between devices, not as fast as oneMesh network in this roaming.  

@TinH 

In below network topology: 

TP-Link WiFi router (oneMesh compatible) - Wired Powerline Adapter ............(powerline).........Wireless  Powerline adapter ...........(powerline).........Wireless  Powerline adapter

Wireless powerline adapters and Wi-Fi router are in the same Mesh network.

OneMesh is built between router and wireless adapter through wire & powerline, not by wireless.