I'm in a very similar situation OP. My apartment only comes with 1 spot. My GF and I both have cars, so one of us parks in the spot, the other on the street (residential). The residential street parking has a system in place for these things. It's 2-hour parking 8am-6pm unless you have a permit (which you acquire if you own a house on the street).
We always have 1 car on the street starting around 6pm when one of us comes home from work, and leave in the morning around 8. If anyone left a note like that on my car I would burn it.
Then again, you seem to be taking 2 spots routinely according to the note so you should 100% be more considerate when you park, otherwise you may find your tires slashed. Also consider parking in slightly different spots on that street, even if you have problems with hills. The person living there has a right to be annoyed and communicate that with you, but they do not have a right to own the street parking.
We always have 1 car on the street starting around 6pm when one of us comes home from work, and leave in the morning around 8. If anyone left a note like that on my car I would burn it.
Then again, you seem to be taking 2 spots routinely according to the note so you should 100% be more considerate when you park, otherwise you may find your tires slashed. Also consider parking in slightly different spots on that street, even if you have problems with hills. The person living there has a right to be annoyed and communicate that with you, but they do not have a right to own the street parking.