Let's say that you have Pisces intercepted on a house (Aquarius is the main ruler of that house). If one have, for example, Uranus on Aquarius, would that planet affect both Aquarius and Pisces?? Or only Aquarius is affected??

The same with aspects: would them affect both signs or only the one where the planet is posited??

No in that example Uranus would only relate to Aquarius. Aspects too or from the planet would only relate to the signs the planets are in. An interception in that area of life a person has hidden traits of that sign that can come out after a person is more comfortable often but are not immediately noticeable. Intercepted planets are more weakened their function is kind of there sometimes not others or is more difficult or held back in some way depending on the planet and how prominet it is i think doing planetary squares helps here.

This is the way I understand it. I have Pisces intercepted first house so maybe I am a good person to answer this there is no planet there though. I come can be quite detached and freedom loving yet sensitive and empathetic at the same time. I can be quite deep but tend to be weird or think outside the box. The water aspects of this I don't tend to show immediately to people until I know them more then it starts to come out.