So I want to get a double up front and I currently have:

SRAm X.0 shifters
SRAM X.0 rear der (medium cage)
LX front der.

Can I simply fit a double (and maybe bash), set the limts on my front der. and maybe shorten the chain a bit and away I go, or will my SRAM shifters not work on a double?

you don’t need to do ANY (really) of that. Just remove big ring and fit a bash… setting the limit screws are just for people who are too stupid to remember they only have 2 rings now, not 3. In 4 years i’ve never tried to change up into the big ring i don’t have fitted.

Leave the chain, leave everything else.

If you are 100% sure that double & bash is the way to go, then shortening the chain will help avoid chain slap & keep good tension in the chain at all times – set the chain length at the tightest point (ie through rear mech & over largest sprocket & middle chainring).

Just be aware that the LX front mech doesn’t lift as quick as a specific double one would do, so although it clears when going from (for instance) 22-32, it may not clear a bigger middle ring (36 or 38) that as more commonly found on double and bash setups.

I have an SLX double front in my parts bin if you want it… £15 posted, in great condition…

For the 3 seconds it will take I would adjust your stop your high limit stop screw too. You may never do it, but it’ll be a pain if you do it by accident in the dark or sommat.

My triple mechs work fine with double ring set ups with a 22 .// 36 chainrings

also thinking about getting a new double crankset. What sizes are sensible if going 2×9? Imagine i would want the larger ring slightkly bigger than my current middle ring? (44-32-22).

Is there anything stopping me getting the Truvative XO 2×10 chainset?
http://www.sram.com/truvativ/products/truvativ-x0-10-speed-crankset

Nope, nothing at all. Ring sizes are personal. I liked 28/40 with 2×9, but it sounds like you would rather lose the the top end than the bottom, or a bit off each.