That is unfortunate situation. What makes your matter worse is that these two parts come from quite old set (OP mentioned 8466). Aged parts tend to have stronger clutch power than the newer parts. Even if these two parts were new they would be pretty hard to take apart. In this situation I think the only solution is taking sharp knife (like x-acto or utility knife) and gently go around the edge where two parts get together to try and make them go apart slightly. I would recommend using the edge of the blade, rather than the sharp point first. Don't try to disassemble them with one go, but rather do it gently around with your knife until you get some progress.
Alternatively, if parts cannot be disassembled, you have two more options:
- Get in touch with LEGO Support and complain about it. It is highly likely they will just ship you new parts, despite set being so old.
- You can buy them yourself. Parts are:
32017 - Technic, Liftarm 1 x 5 Thin
32124 - Technic, Plate 1 x 5 with Smooth Ends, 4 Studs and Center Axle Hole