From my pov it's pretty easy to delegate this with the following logic. Is the it a weapon? Does it do anything beyond granting an extra attack if you have 2? If the answer is yes to both of these questions, then it's a ccw that grants a special effect, and you can never have more than one such effect.
Is the scorpion chainsword a weapon? Certainly, it grants an extra attack if you have 2 and says it is. Does the scorpion chainsword do anything beyond granting an extra attack if you have 2? Why yes, yes it does. It gives +1 str to the model's attacks. The answer to both questions was yes, therefore you can't double on on special weapon effects. GW is notorious for having some vaguely opposed wordings, it's much better to use the general rule that they're obliquely referring to to govern all situations not explicitly worded as being an exception to the rule.
If you want to rationalize it the scorpion chainsword is made with advanced eldar engineering such that it is so deadly that it adds +1 to the model's strength when rolling to wound (not really increasing their own strength but rather allowing them to wound easier). So it's a weapon that must be used to be effective. It's not like the chainsword is really heavy so they do strength training in order to wield it. That would be reflected by having a base 4 strength.