This is what I read in the last version of the rules, available here in the files section. Especially the last paragraph is interesting and made me doubt my original understanding of the rules.
10.12 Activation. The total number of units that a leader may Activate for movement and/or combat is equal to the combined Leadership ratings of the stacked leaders, up to a maximum of eight combat units. Note that leaders may be stacked with a number of units greater than their Leadership ratings, but may only Activate a number of units equal to or less than their combined Leadership ratings.
A leader may activate more than one army per activation, by moving from army to army, as long as the leader does not exceed his 8 MPs.