Getting the color of a specific pixel in a Custom node?

I’m working on implementing box blur in a post-process material. The specific function requires me to use two custom nodes, since I have to run over all the pixels on screen twice, in order to blur it horizontally and then vertically.
in the first custom node I just refer to a scenetexture node, and get the pixel colors I need with SceneTextureLookup. But in the second custom node, I need to refer to the output of the first custom node, and get specific pixel colors from that.

Does anybody know a way of doing this?

Thanks in advance!