The short version

Don't.

The long version

( I've not gone into too much detail, I figure the only people who'll stumble across this are either considering the same solution, or troubleshooting their own attempt )

I had a Raspberry Pi 2, it's a "2+" I think, running Xbian. Xbian is a pre-built version of Kodi, the popular media player that used to be called XBMC. No X server is used, Xbian turns your Raspberry Pi into a media player with relatively little effort.

Having acquired a couple of USB screens over the years I thought it would be useful to connect one of these screens to the Pi, just so something like BBC News 24 or the NFL Network could run in the background to the side of my main monitors, or a Twitch channel.

So I connected a Mimo USB UM710 monitor and rebooted the Pi. This came up as a green screen, which means that the udlfb driver has loaded; and from the command line I can see that I have "/dev/fb0" and "/dev/fb1" - meaning that two framebuffers are available.

However I couldn't find any way within the Xbian interface to direct Xbian to use /dev/fb1, nor any kind of option to specify this in any of its configuration files.

I tried using the con2fb tool to redirect a different console to each framebuffer, directing tty1 to the USB monitor, in the hope that Xbian was starting on tty1 ... but still running "kodi start" from the command line brings up Xbian on HDMI.

I looked at somehow disabling the first framebuffer, but to no avail; the relevant bcm2708_fb driver is part of the kernel, and there's no way to stop it being used. Also I don't know if that functionality is required to generate that graphics that are then sent to the USB monitor using the udlfb driver. I expect that a Raspbian kernel can be compiled that doesn't include this functionality, but I decided that for a relatively simple system, which I'm trying to use in an "plug and play" way as possible, compiling my own kernels was a step too far, especially as I had no idea if the solution would work or not.

Also, ideally, I would be able to switch this device from using the USB screen to an HDMI screen with a few commands.

( As a side note, if you're looking at this in general it's worth researching the "chvt" and "xbmc_send.py" commands online )

On further research it turns out this is a common issue for people trying to extend their use of a Raspberry Pi.

That research did lead to a couple of possible solutions, these are framebuffer copiers, or mirrors, that copy of the output from framebuffer to another. While not ideal, this could work.

Firstly I tried fbcp but that just didn't work.

I set the Xbian resolution down to 480p to match what the USB screen was capable of, but this didn't make a difference.

So I moved on to raspi2fb instead.

This worked up to a point, showing the output of the first framebuffer at the right resolution, and at something like 25 frames a second. While slightly jerky this was more than enough to satisfy my requirement to keep an eye on the channel. Kodi's BBC News 24 plugin worked fine, the NFL Network worked fine at a low enough resolution ... but both the Twitch and YouTube plugins would crash the entire system. As far as I can tell it seemed that if I attempted to display anything above the resolution supported by the USB screen the Pi would just crash and need to be manually restarted. Also the system was now a little flaky in general.

I tried both 1.0 amp and 2.0 amp power supplies with the same result.

In the end I gave up, and decided I'd try something else to get Xbian on the USB monitor.

However having disconnected the USB screen, and tried using the Raspberry Pi on an HDMI monitor again, it's crashed after a few minutes. I'll be seeing if there's some kind of software diagnostics I can run to spot any obvious problems - it feels like something the community will have written already.

So in the end I have a Pi that appears to be broken in some way, possibly a result of how many USB devices I plugged into it at once - suggestions for easy ways of running hardware diagnostics are welcome in the comments below.