You could hold separate Shogi and Xiang Qi tournaments simultaneously.
Which raises the question: are the people likely to win these tournaments
particularly interested in playing in a multi-variant tournament with a
'western bias' to its games? If not, then around the same time as the
multi-variant tournament we could hold a Combined Tournament: a four
player double round robin made up of the top two Shogi players and the top
two Xiang Qi players . Each player would play one Shogi game and one Xiang
Qi game against every other player.