Who said you can't? When you critpunch(I like that term!), you're punching, with all the rules therein. I haven't seen a rules thread about it, but I also don't see the need for one.
A critpunch is a punch.
You can double-punch.
Ergo, you can double-critpunch.
Now if you extend the blade and use it as a melee weapon attack, then yeah, you can only attack with one at a time. On the other hand, that might sometimes be desired, such as when attacking something really fast and you want the to-hit bonuses, or something lower than you and thus unpunchable or a 'mech already has weakened legs, but the odds are low so you don't want to risk falling after missing a kick...my favorite part about the blade is the flexibility. It gives you many options, even if some of them only come up in very narrow circumstances.