@Carl I think I already have the HS210 on the power side. I remove both switches and separate the wires then I tested which wire has the power and that's where I installed the HS210. Also, that...

@Carl this setup will make the mechanical switch useless. I was able to connect one HS210 with a mechanical 3way switch with full functionality, but when I use the other HS210 in another 3way switch...