i have 2 pair of speakers at 85db and 89db, will a 40watt amp drive them?
Most anything will drive most things, the question is can you get the dynamic range you require.
Theoretically, the 40W amp will give an extra ~16 extra db before clipping. In real life, a bit less. Theoretically, you can drive the speakers at low volumes. Theoretically the dynamic range of the system with 1st spkr pair maxxes at ~100db and the second pair at ~105db spl.
In practise, the short answer is, you're probably better off with a higher power amp OR more sensitive speakers!
There are many other parametres involved that limit the ideal situation -- one of these being how well is the amp dealing with the speakers return, another is the current capability of the amp's PS (because the speaker is NOT a constant 8ohm load at all frequencies).... etc