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You can stack them but....
1. Narrowband filters:
The UHC includes the H-beta & O-III lines whereas the O-III is just that, O-III only and very little either side. Therefore if you stack an UHC with an O-III you get an O-III with less transmission than using a single filter since each filter has less than 100% transmission.
2. Coloured filters:
Once again the transmission curves need to be looked at and if there is any overlap you will get some transmission of a mixed colour but this is not ideal as using a single filter of the same colour as the transmission will be better for the single filter. I think too that for 'scientific' planetary observing it would be good practice to use standard filters such as Violet 47 or Red 25A or light Blue 80A or 82A so that other observers know exactly at what wavelengths you observded any particular planetary feature most prominently.
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