Like I did with Lydia, I gave him stuff until I got the error message and my inventory grayed out. I then took everything Faendal was carrying, bringing my carry weight up to 491. My carry weight at this point was 180, which means Lydia maxed out at 234. Once this occurred, everything on my side of the trade menu greyed out. I then gave her stuff until the game refused to allow me to give her anything more, producing the error: I first took everything Lydia was carrying, which brought my carry weight up to 414. To test this, I tried testing with two followers: Lydia and Faendal.
