Put a coat of fresh Tamiya extra thin cement on the joint. This will resoften the plastic and previous cement and you should be able to carefully work them apart while the plastic is still soft. It may look a little rough after you get it apart, but you can fix it by sanding off the messy part before reglueing it.
Remember, this stuff is not really glue, rather it is a plastic solvent, so a reapplication can undo the process.
I, too, use Tamiya thin cement (or MEK when you can find it) and haven’t had much luck using it to separate parts, particularly long seams. I’ve resorted to the CMK or JLC razor saw and then repair the damage with lots-o-putty.