Its come up a few times and people asked me to post about it, so here it is.

Separate pools of power are just that, separate. You can't combine pool from one pool with another. To avoid confusion:

Spiritual healer/Devout Priest/Arch Mage/Similar add power to your main pool
Normal power stores add power to your main pool
Regening power stores form their own pool.
Vessel of spirits and similar classes/abilities that are a "separate pool" are separate.

Having more than 2 pools of a specific power type needs them to say they all work together.

So as an example:
You have 3 power left in vessel of spirits, it means you can only cast miracles upto 3rd level. You could not use 3 power from your main pool and the 3 from your vessel pool.

Hopefully makes sense. If there are any questions please use the ask the GSM thread.