I have connection problems with reinforced concrete walls
Settings > Hub Mesh. Works great. You'll want both hubs on ethernet so you don't introduce a Wi-Fi connection into the mix, obviously.
Note that Hub Mesh, above, is just a way to share device events and similar over your LAN. You will still have two (or however many) hubs with their own Z-Wave and Zigbee networks (if you are using those radios). Stated differently, if you are hoping to use one hub to "extend" or "repeat" for your meshes, i.e., making one larger Z-Wave or Zigbee network, it won't -- you'd need to try actual Zigbee or Z-Wave repeaters for that, and luck may vary depending on distance, building materials, etc.
So, this could be what you're looking for, and it can indeed help with situations like that -- just wanted to be clear about what it is and isn't!