It is common for dulcimer hammers to be double-sided, with one side being unaltered from the wood that makes up the rest of the beater, and the other covered with a thin layer of felt. The felt side will have a warmer sound, with very little attack in comparison to the very bright quality of the wooden side.
For reference, listen to this recording of me playing Cassie Wieland’s Heart. The piece begins with the felt before switching to the wood.
In this piece, the switch is achieved by way of fermata. It is entirely possible to switch sides in the midst of playing, but it takes a second. Going constantly between the two from note to note would not be doable, but allowing for a change via a short rest in one or both hands can allow for a quick change.
It can be assumed that a passage should be played with the wooden side of the hammer unless otherwise specified. However, it doesn’t hurt to include a reminder. Simple boxed notation is very handy for knowing when to switch sides.
If the player is to be using the wooden side in one hand and the felt in the other, consider using separate staves, opposite facing stems, or whatever the clearest notation happens to be. When doing this, it is extremely important that you understand the layout of the instrument, as this technique could end up demanding some unachievable cross-overs and such. For reference, check out the 16-15 tuning setup.
You can download my sample pack made with my own 16-15 hammered dulcimer here! (Hard mallet samples only)