Unclear and probably allowed, and of course it comes at a big karma, nuyen and Focus Addiction price. The main reason why a GM could disallow it, is that it's all too easy to get around a big Availability check through this. A Force 8 Qi Focus has Availability 24, so someone who gets that Focus has to go through a much bigger Availability check AND he can't pick a lower Force to have the Focus active on. So allowing this stacking would be unfair to the people who go for the higher Force, and would make the creation of high-Force Qi Foci quite the bad market.