Well, I think, this can be calculated, so lets do it.

First, we need to understand how the damage is calculated. For this SolemnWolf has already provided a formula in this thread: Damage Calculation

In short, it should be as follows: Damage ≈ 100 x ( θ x Att / Def ) ^ 1.35

Θ here is a random number between 0.606 and 1.649. To simplify, I will just ignore this random factor by replacing it with 1.

Next, lets assume that our target has a defense value of Def=750.

Athenas attack score without emblems is 737 and she does 305% on target (Att=737x305%=2156.35) and minor damage to nearby enemies.

On her own her average damage on our target should be around 100 x (2156.35 / 750 ) ^ 1.35 = 416. If I am not mistaken, her minor damage should be around 50% of this value = 208.

After Fridas -54% elemental defence debuf Athenas average damage on our target should be around 100 x (2156.35 / (0.46x750) ) ^ 1.35 = 1187

Fridas attack score without emblems is 733 and she does 290% on 3 targets (Att=733x290%=2125.7).

On her own her average damage on our target should be around 100 x (2125.7 / 750 ) ^ 1.35 = 408.

After Athenas initial -41% elemental defence debuf Fridas average damage on our target should be around 100 x (2125.7 / (0.59x750) ) ^ 1.35 = 832.

So, if all we care is the main target, then Frida+Athena should do around 408+1187=1595 and Athena+Frida should do around 416+832=1248 damage. It is not this simple, however – if you do Athena first, then get her buff to -65% with some tiles, then fire Frida, the results should be even much better (416+tiles+1684=2100+). Without the tiles, Frida+Athena may win.

Also note, these values will differ depending on the targets initial defense as well as that random factor in the damage formula which I ignored here.