...attatched the two pieces. If it is actually an adhesive then you could POSSIBLY find a solvent that disolves the glue but not the plexiglass.
That being said the only glue I ever used for plexiglass to plexiglass work, back when I worked with it, was a glue which was not an adhesive(sorta like some PVC glues). What it was composed of was some colored(or clear) plexiglass dissolved in a solvent. When you applied it between two pieces of plexiglass it slightly melted/dissolved the two touching surfaces. You then clamped them together with slight pressure for about 15 minutes after which the two softened edges would have fused together.
If they used a plexiglass glue like the ones I used then you won't be able to seperate them cleanly, at least not in whole pieces. You could use a tool(saw, rasp, file, or grinder of some sort) to remove almost all of the piece you do not want and then sand the remainder off. If you do this though you will have to go through the process of sanding plexiglass smooth and pollishing it if you want it to look good. This takes alot of work cause you have to work all the way down to wetsanding with emory paper then using cloth and polishing agent if you do not want the plastic to look scuffed but it will do the job.
If you try the solvent route then make sure it is not a solvent that dissolves plastic.
Also remember that while plexiglass does not shatter from impact type force it shatters pretty easily from compression or torque so be carefull trying to wedge something between the pieces that you do not shatter the part you want to keep.
Sean.