Alt and X are separate events, there is no single event for the two keys as a combo.

What you want to do is possible, but there are caveats.
See my comment in this other thread: keymapper. How to bind a shift+key event ?

Thank you, I will try this when I get home.

EDIT: That worked perfectly, thank you!

After saving the mapper file and restarting DOSBox you may find that the key combo is no longer working. That's probably because the binding order is reversed after the mappings are read, as I mentioned in the other thread. A workaround is to manually edit the mapper file and put the modifier (Alt) event before the key (X) event; but keep in mind that the order will be lost again if/when DOSBox writes out the mapper file.

ripsaw8080 wrote:

After saving the mapper file and restarting DOSBox you may find that the key combo is no longer working. That's probably because the binding order is reversed after the mappings are read, as I mentioned in the other thread. A workaround is to manually edit the mapper file and put the modifier (Alt) event before the key (X) event; but keep in mind that the order will be lost again if/when DOSBox writes out the mapper file.

Hmm, the setting is sticking so far. The order doesn't seem to matter.

The game is X-Wing, the 1994 CD version. I wanted to be able to use the rudder (twisting of the stick) to do a roll, which you do by pressing the second button while pressing left or right. So I mapped the left twist to button 2 + left key, and the right twist to button 2 + right key.

Binding order is a concern for modifier+key combos like the Alt+X you mentioned in your initial post, but probably not for the controller+key stuff you're actually doing.

ripsaw8080 wrote:

Binding order is a concern for modifier+key combos like the Alt+X you mentioned in your initial post, but probably not for the controller+key stuff you're actually doing.

I see, thank you.