I'm sorry to say, there is no easy solution.
The problem is, that Radeon cards are not supported by fully free drivers.
Ubuntu uses a kernel with so called binary blobs, that are loaded by the
drivers during runtime. In the case of Radeon cards these blobs include what
is needed to fully utilize all the features, like 3D and 2D acceleration,
multimonitor support, etc.
Because Trisquel respects your freedom and doesn't include any non-free code
in it's kernel, all the hardware that needs the blobs to run correctly will
show issues. I had to put up with a X850XT Radeon for a long time and hated
it.
The sad truth is, AMD won't release a fully free driver or the documentation
necessary to write one, because it will allow users to easily bypass the DRM
they outfit the Radeons with.
You have two options that will make Trisquel work 100% with your dual monitor
setup:
1. Buy a GeForce 9500GT from libre.thinkpenguin.com. It is the one GeForce
with best support by the reverse engineered Nouveau driver. Don't buy a
9500GT somewhere else, because the chipsets in use may vary on different
9500s, without notification on the package. 25% of profits from this shop
will go to Trisquel if you use the link I provided, or the link that is
provided on the right side.
2. Buy a motherboard with an integrated Intel X4500 graphicscard, or a
motherboard with the support for the new Core i CPUs. The built-in
graphicscards from Intel run even better, there is a fully free software
driver provided by Intel themselves.
You could also use just one monitor, but I don't suppose that's the answer
you were looking for...
