Double-sided printing is possible. However, it can be problematic with letterpress because of the show-through of the impression.  We can control how hard the type is impressed into the paper, but the paper is squeezed between the plate and the tympan, so any pressure applied to one side inevitably affects the other.   In general, double-sided should work with artwork that has light coverage where the elements don’t back each other up precisely.  Otherwise, compromises need to be made to balance the amount of impression with the amount it shows through on the other side.  Using heavier papers, or duplexing two papers together, can help, but doesn’t completely solve the problem.