My first question is what are you trying to accomplish by adding two monitors to your system? Do you feel it is lacking in some way, or are you just trying to get a better sound? Focal make some fantastic speakers, with drivers that are second to none. But like everything else, it comes down to preference as much as anything with their sonic properties. I like Focal as much as I do Dynaudio, even though I exclusively own Dynaudio speakers. If given a choice and a limitless budget, I would go with Focal Utopias over Dynaudio's Evidence Masters from both a sound and aesthetic perspective (I have heard both, simply amazing!).
As far as overworking your amp, it is possible. Tpreaves is right, only your amp manufacturer can accurately answer that question. I will say that Dynaudios are notoriously power hungry and will make an amp work hard trying to power their 4 ohm load. Focal bookshelf speakers are slightly more efficient, so you might want to consider using the 806 0r 807 instead of the Dyn 140 if amp load is a primary concern.
Have you considered upgrading your 826? If you are looking for a fuller sound you might consider the Focal Electra's especially the 1027BE's (if you can afford them, they are pricey).
