So, the math is done:
Assumptions: As I am trying to emulate how much damage is inflicted, I weigh the likeliness of hits with the amount of hits. Neither is attacking and the unit consists of Commander Farsight, a Fusionblade Commander and two Shield drones vs. a regular (read non-FW) Imperial Knight.
1. Farsight attacks:
4 attacks x 2/3 chance to hit (conveniently the number of hits negates the chance potential, so this easy formula works) x 1/6 (glancing hit)
+ 4 x 2/3 x 5/12 x 5/6 (penetrating hit without explodes)
+ 4 x 2/3 x 5/12 x 1/6 x 2,5 (penetrating with explodes)
= 96/216 + 200/216 + 100/216 = 396/216
= 1,833 points of damage
2. Knight attacks:
3 attacks x 2/3 (the drones low WS) x 5/6 x 1/2 (shield save)
30/36 = 0,8 wounds
3. Fusionmander attacks
4 x 1/2 x 1/9 (glancing hit)
+ 4 x 1/2 x 13/18 x 5/6 (penetrating hit without explodes)
+ 4 x 1/2 x 13/18 x 1/6 x 2,5
= 48/216 + 260/216 + 180/216 = 488/216
= 2,25 points of damage
4. Knight stomps:
1/6 chance of total annihilation
2/3 x 4 x 5/6 x 1/2 (drones save) = 1,111
- 2 drones dead + 17% likeliness of all Tau annihilated
- 4 Hullpoints destroyed
Conclusion: After one turn, both drones will likely be destroyed and with a 17% chance the whole unit will be gone without any saves.
The Knight will have lost 2/3 of his hull points and Farsight will most likely finish him the next turn, if he wasn't stomped to a fine paste on the ground.
It seems to be a valid tactic to engage a Knight with both Farsight and a Blademander, only one of them won't do it. Better than Farsight would be Brightblade plus a Blademander, but the difference isn't that big. If the Tau had attacked, averagely a fifth hullpoint would be gone, but the Knight would survive. If the Knight had attacked, one of the Commanders would be dead, too, so beware of charging Knights.