Rand's Gateways:

People moronically assumed Gateway size had to do with OP strength due to Logain's and Pevara's comments, even though there are direct quotes proving otherwise:

Yet, we see Rand make six Deathgates of this size. From Aviendha in tPoD, we know that other women, like Rand, don't even use their entire strength to make a Gateway. This means you need a minimum strength to make the Gateway, but beyond that you can't increase the gateway size no matter how much more of the OP you add. And this is not even considering things like having Talents in Gateways.

Similarly, we see Mylen do three shields, and we see Egwene weave twelve gigantic columns of fire.

With shielding itself, what this means is clear: just because you can shield two women of strength X doesn't mean you can shield one woman of strength 2X. Especially when said women weren't even already holding the Source.

It is also clear that the Aes Sedai have a misguided idea of splitting flows, as we can see from the Companion:

However, here is what Egwene thinks:

See, Egwene assumes that because it takes more effort to do two things at once, and so on, it must be a matter of strength to match what Rand did. But she herself is proof that isn't true. And if we need more, the Companion says Aviendha could perhaps weave as many flows at once as Nynaeve, but less than Elayne. Egwene seems to be off the charts for this comparison.

It is fairly clear what happened. Egwene, we see, tries splitting her flows naturally as early as book 3. But tSR, she can do 4. We can surmise what happened next: she saw Rand do 12, so she practiced to push her own limits to the maximum. Turns out her maximum was as much or more than Rand's, who himself learned to do multiple things simply because no one told him it wasn't possible.

So I think you are very right to doubt if someone having the ability to shield two people at once means that he/she is stronger than those two combined.

Here's another sign of how little splitting flows affects shielding:

Egwene, in LoC, is able to shield Shalon, and at the same time, lift three women to a height of ten or twelve paces which is 30-36 feet, above the river. Per Siuan:

So Egwene is doing what was the upper limit for Siuan in weight (assuming none of the Sea Folk woman was much heavier or lighter than Egwene), AND holding a shield on a not at all weak woman... AND she is lifting them 30+ feet in the air!

Do we really believe that the difference in strength between Siuan's original level (13) and Egwene's ( 8 ) is enough to shield someone of Shalon's strength AND boost how high Egwene could lift them? No way in hell.

This means that once Egwene wove the shield on Shalon, she was able to redirect much of her strength on her other task. Her her own PoV, all the strain was in lifting them as high as possible:

The conclusion is obvious: holding a shield on someone isn't very hard. And more importantly, if you trained yourself early enough, you can do insanely impressive things while holding shields on someone. Heck, even when dueling to shield someone as strong as herself, Nynaeve splits her flows at least 3 other ways, and likely 4, from what we see from her lesson to the Sea Folk in Winter's Heart. Talaan herself splits her flows six other ways.

We know shielding someone holding the Source takes strength, and once the strength differences between you and the person you're trying to shield are not that great, your chances are never a 100%. Yet, both Nynaeve and Talaan find it possible to split their flows many ways?

To me that shows that even there you don't have to use all your strength. Heck, the shields Nynaeve and Moghedien threw at each other wasn't their only weave either. They both had to make another to block each other's shields. What does that say? To shield someone exactly as strong as you, but already holding all of the OP they can, you needn't use all your strength.

How much less do you need when someone isn't already holding the Source?