There are several reasons why you might want to have multiple MIOS Engines in one home.  You might want a MIOS Engine to be physically located next to an alarm panel, for example, so you can connect it to the alarm panel with a serial cable.

One of the most common reasons is to improve the reliability of Z-Wave devices.  Z-Wave devices are 'mesh networks', meaning if device a needs to control device b, but they're not in direct range for the wireless radio signal to reach, then other Z-Wave devices in between a and b will act as relays.  However, there are limitations to this, such as a limit to the number of hops a message can make, the signal may not travel through some barriers like brick walls, you may have a 'dead zone' area in your home where there are no nodes or other interference, or you may have separate areas like a main house and a guest house.  Z-Wave is quite reliable when you have a lot of Z-Wave devices in a small area so the number of hops is kept to a minimum.  So, you may want to put one MIOS Engine in the main house, another in the guest house, or divide the main house in 2 halves with a MIOS Engine right in the middle of each half so that most Z-Wave devices are in direct range of the MIOS Engine.  There is no limit to the number of MIOS Engines you can have in your home and, so long as they are all connected to the same home LAN (ie Ethernet network), multiple MIOS Engine's can be bridged so they act as one.

The way bridging works is that you go to one MIOS Engine and "import" or add the other one.  So, MIOS #1 can import the devices from MIOS #2 and MIOS #3, and that way on MIOS #1 you will see all devices from all 3 MIOS systems together as though it was one system.  You can create scenes and events on MIOS #1 that control devices from all 3 MIOS Engines, and you can attach events to those scenes that are trigged by devices on all 3 MIOS systems.  When you control MIOS #1 from the web interface or a mobile phone interface, you will have control over all devices from all 3 MIOS Engines.

There are no limitations on importing other MIOS systems.  So, technically if you had 3 systems, #1 could import the devices on #2 and #3, and #3 could import the devices on #1 and #2, and in that way both #1 and #3 would have all the devices.  This might lead to confusion, though, particularly if you have scenes and events on both #1 and #3, so you might see things happening and not know which MIOS engine is actually doing what.  To keep it simple, the best solution is to generally think of one MIOS Engine as the "Master" and have it import the devices from all the other "Slaves", and then create scenes and events only on the "Master".  You won't need to do anything with the "Slaves" except include the devices and modify the device settings for those devices because on the "Master", the 'setup' icon on the dashboard  will only let you change settings for the Master's native devices.  To change settings for a device, such as Z-Wave variables, you will need to use whatever MIOS Engine included those devices natively.

To import the devices from another MIOS Engine:

• Click the button from the dashboard
  
• Scroll to the bottom of the Add/remove device window
• Click the next under "Add control over another UPnP device, such as another MiOS system."
• Click add for the MiOS system that you want to be able to control from this MiOS
• Wait a few minutes for MiOS to scan the device's from the other MiOS's Z-Wave network
• You're done. You've successfully bridged two Z-Wave networks with MiOS