To my understanding, crystals can affect you by changing your frequency only when they have the "dominant frequency". So if i carry two different pocket stones at once, will one of the crystals be ineffective or will they both work simultaneously?
There are many, many different layers of energy (frequencies or vibrations, as they are sometimes called), so more than one can be active. There are a plethora of things about crystals that are path specific, but in general, you can work with more than one at a time.
Usually I've seen that it is most beneficial to carry two stones that compliment each other's healing purposes. This is honestly a tougher question. Some crystals and stones don't even "like" each other and will not "play" together. I highly suggest hibiscus Moon on YouTube she's great at what she does and that is "Crystals."
Good example is amber and jet - they compliment each others energy. This 'magical marriage' is because of the interesting qualities regarding witchcraft. Amber, 'solidified sunlight' empowers the witches magic, while jet grounds. Hence some 3rd degree witches and priestesses wear this combination.