Jesus is talking to a group of people about what they as individuals should and should not do. All occurrences of "you" and "your" are plural. (See: [[rc://en/ta/man/jit/figs-you]])
This is a metaphor for total secrecy. Just as hands usually work together and each can be said to "know" what the other is doing at all times, you should not let even those closest to you know when you are giving to the poor. (See: [[rc://en/ta/man/jit/figs-metaphor]])