My group's having a debate when spliting dice pool such as a Adept attacking two targets in one pass with his weapon foci or a mage with a power focus splitting his dice to cast two spells.

Is the foci's bonus dice added once then split, or added to each split dice pool.

The intent is added, then split.

Alright I thought it was a bit crazy to say split the pool to 10 dice then add the power from the focus.

The intent may be to add the foci dice before the split, but by RAW, splitting your pool (such as attacking multiple targets) creates multiple new pools. Dice pool modifiers (such as foci & specializations) all apply after splitting your pool.

Actually, while attacking multiple characters is explicitly split before dice pool modifiers are added, multiple spell casts do not say they are. This heavily implies that they have all modifiers applied to the pool before it is split, as otherwise, it would have language similar to the attacking multiple characters.