I was think of a stunt that would allow a twinborn compounder to burn both a metalmind and their regular allomantic ability at the same time. (i.e. a bronze compounder able to compound wakefulness while burning regular bronze to seek, making good sentries.) this ability is so difficult to handle (even as a stunt) that they can't use other allomantic stunts while doing this. (maybe, if this is already a thing, there can be a stunt that allows them to use the allomantic stunts along with this ability.)
I don't know if a compounder can actually do this. if I remember correctly, miles stopped burning his goldmind so that he could burn normal gold to become captain hindsight.