Which actually makes a lot of sense if you think about it - saidin and saidar are different forces, and react differently. Trying to restrain one with the opposite force may lead to a weaker containment field.

Even the Ashaman, on average possessing more raw power than the average Aes Sedai, when we see the strongest of them, have only been shown to hold one Aes Sedai - Rand himself is worried that he might not be able to handle more than six or seven Aes Sedai unaided; this can be interpreted to mean resist being Shielded and neutralize them rather than Shield and hold them all.

Shielding is a technique that has quite a few flaws, but many of the flaws involved are ignorance and lack of dexterity. I happen to think that the Aes Sedai techniques are so lazy in the form of a Shield projected straight ahead rather than utilizing Sea Folk techniques, that may be why they are quickly defeated by more aggressive channelers who'll whip a Shield around their guards and use the environment against them.