The 11'6" 8/9 GLX was designed for single hand rod lines. It is really a single hand rod that can be cast 2-handed because of the short rear spey grip. The rod is very close to the 11' IMX 8/9 I own, the main differences being the slightly longer length that was added to accomodate the short rear spey grip and slightly longer rod grip. It casts the same lines as my 11' IMX, namely the 8wt steelhead/salmon lines, 8wt DT, 9wt standard WF lines, and either 9 wt or 10 wt shooting heads. I use mine with an 8 wt RIO steelhead taper, 9 wt RIO single hand Windcutter, or 38' 9 wt floating shooting head for bass bugs.
I would not use a spey line on the rod because it would greatly overload it. Remember a 9 wt line is 240 gr (230-250 acceptable range), and a 10 wt is 280 gr (270-290 acceptable range) in its first 30 ft and that is what the 11'6" GLX 8/9 was designed for.
A spey line, even the light 5/6 Windcutter spey lne is 365 gr for its 48' belly and the 6/7 Windcutter spey line is 455 gr over its 48' belly. This means that the 5/6 Windcutter might load the 11'6" GLX (it will be badly underloaded at distances shorter than 35 ft) and will be very overloaded with the 6/7 Windcutter if casting the complete belly. Which leads to the question: Do you want to have a very nice 8/9 single rod that can be cast 2-handed, or do you want to have an overly powerful and badly overloaded light trout spey?
I'd stick with the 8 and 9 wt lsingle hand lines it was designed for.