The configuration you describe, 2 power modules, 2 oscillators, 2 mix, and 1 speaker is safe.
In general it is possible to use multiple power bits in a circuit, We don't usually recommend it unless you absolutely need the extra power--for example you are driving multiple motors, you have a very long chain of modules, or you have multiple cloudBit modules in a circuit. In those cases, add the power modules as close as possible to (that is, as few bitSnaps away from) the modules that need additional power. (A common setup that I use is: Power > AND > DC Motor. The other input on the AND is then controlled by the rest of my circuit, and I don't have to worry about the DC motor not getting enough juice).
If you are ever unsure, just post your circuit and we'll let you know.