- Possible to combine?
I don’t see why you couldn’t combine both types of sensors into a single device. The key idea is to allow sufficient access to the air for each sensor.
- Restrictions if together in one housing?
The only restrictions I can imagine would be that you have to design the housing in such a way that they both get good access to the air they are intended to measure. This is a mechanical, fluids, and thermodynamics design problem which should be solvable through modeling and/or experimentation. In general, the larger you make the housing, the easier it should be to arrange the two sensors in a way that they do not interfere with each other.
- Estimate number of sensors needed?
The answer is completely application dependent. It would depend on the size of the area to be “sensed”, how much air movement/mixing there is within that area, and how much resolution you need within that area.