They practically do the opposite of each other. Smoothing tidies up jitters and subsequently increases aim assist, while steady aim basically creates jitters. So as said above they'd cancel each other out. So rather than increasing/decreasing both, use one or the other.
If aim assist is too strong then reduce smoothing; or alternatively, if you're already on the minimum amount of smoothing required to eliminate mouse jitter, then add in some steady aim. Low amounts of steady aim (less than 3, or more depending on what sync you use) aren't visible to or felt by the player, but the game definitely feels them and it will reduce the aim assist. If you feel like you want more aim assist then you won't want to be using steady aim, whereas adding smoothing will increase aim assist, generally.
