Hi, first time posting! Got here from searches on rotten wood and sistering, etc. Good times!
There is water damage in the corner of my kitchen from bad fascia and siding flashing work. I've got the fascia and siding fixed--now I've been tackling the inside.
You can see what I've done on this video, but I have questions about what I should do with my corner post, which is completely rotten at the bottom.
I agree with bj. My suggestion would be to cut the post out at the bottom of the second of the sandwiched 2x4's, then make the replacement piece of PT 2x4's the length needed to fill this in snugly. Good Luck, David