'3 out of 5' star review on a well known on-line retail site that initially sold books ...

The review was for the double-entry ferrules, from a well-known (and as far as I know reputable) brand of wire end ferrules and their crimp tools, as follows:

Sizes don't match!

Well made but the 2.5 mm2 terminal is too tight to be used for 2.5mm2 Twin and earth cable.

Now, I know it's likely to be possible to actually deform the two solid round 2.5 sq mm csa copper cores into a single square or hexagonal profile with an area of around 6 to 7 sq mm (with quite some force - well in excess of that usually applied to ferrules), but .... you'd actually need a 3.8 mm minimum inner diameter tube (equivalent to just over 11 sq mm area) to guarantee to get two 2.5 sq mm solid round conductors in side-by-side !

So, we sit back and think about how obvious (to us) it might be that you would never easily get 2 round conductors with 2.5 sq mm csa if you had an approx 5 sq mm csa tube ... well, it's a little over 5 sq mm, actually between 6.2 and 7.3 sq mm, as the diameter is minimum 2.8 mm and max 3.05 mm for the twin 2.5 sq mm ferrule, whereas the minimum diameter for 2.5 sq mm using formula for area of a circle is 1.78 mm (incidentally, from BS EN 60228, the maximum diameter of 2.5 sq mm class 1 conductor is 1.9 mm), but again ...

Sadly, that reminded me ... I just happen to know it will be a very tight squeeze, or perhaps not quite possible, to get two of the absolute maximum diameter of a 2.5 sq mm 7-strand class 2 stranded conductor to BS EN 60228 into that 2.8 mm (minimum) diameter tube. In general it will be OK for 2x2.5 sq mm Class 2 7-strand in the majority of ferrule tubes (assuming a standard deviation across each of the 2.8 mm to 3.05 mm inner diameter range of the ferrule, and 1.78 to 1.90 mm diameter range of the diameter of the solid cores of the 2.5 sq mm) ... but not at the extreme of the thinnest ferrule and thickest permitted 2.5 sq mm solid round conductor.

So, on that particular day when everything's going wrong and you are struggling to get those two Class 2 stranded conductors into the twin ferrule, either strip back, and/or pick up another ferrule ... and just perhaps the slight changes in diameter will resolve that problem.