Your interpretation that it means "combined" as opposed to – as she says – "together" relies on her thinking of something like linking, where, irrespective of loss, the "combination" has access to the combined raw power of both. In this interpretation, (A+M)<E.

What she actually says, "Amys and Melaine together", is not represented by (A+M) vs. E, but rather by (A vs. E)+(M vs. E), an altogether different dynamic.

Consider, for instance, that Moghedien defeated both Nynaeve and Elayne in Tanchico (who “combined” well exceed her strength). Liandrin and Rianna defeated Nynaeve in Tear, and Amico and Joiya defeated Elayne and Egwene, in both cases the less powerful victorious. And Mylen is able to hold shields on Teslyn, Joline, and Edesina simultaneously (KOD36), though “combined” they certainly exceed her in strength, as Berowin is able to hold the far stronger Nynaeve (ACOS23). More? Nynaeve can hold a shield on the equally powerful Moghedien, or later Talaan, but she could not defeat either of them by simply slamming the shield down; she had to, like the weaker victors above, create a tactical advantage by distracting them in some way first.

Now, here's where your problem is. We know (ACOS21) that Nynaeve (you say 80) is not strong enough to shield Elayne (50) when she is holding saidar. So how can a 50 (Egwene) overpower the “combined” strength of two women – average, and above average for Aes Sedai, but let’s say they’re both halfway between Daigian and Moiraine, at (you say) 20 apiece, “combining” for 40? Your position is not logically compatible with the text.

What this then must mean is that Egwene does not simply shield them, but defeats them using tactics to create a situation where her advantage can overcome one, and then the other. However well-coordinated, Amys and Melaine are each individually at a disadvantage compared to Egwene; two one-pound axeheads cannot chop at the tree of Egwene’s defenses as effectively as one two-pound axehead.

Once the understanding that Aviendha’s theoretical battle refers to (A vs. E)+(M vs. E) has been reached, you can comprehend that the posited victory does not much more than that Egwene is more powerful than either of them. Which we already knew.
I am saying precisely that, if the statement means what you take it to mean. There are many contradictory statements. You cherry-picked this one because it is the only one which fits your picture of how the "strength ladder" should look.