Tireless March appears to be stacking in Tactical combat in the beta v. 1.29.  Two heroes with Tireless March are giving the army +1 movement each for a total of +2 movement in combat.  I assume three Tireless Marches would give +3 and so on.  On a personal note I'd rather Tireless March give +1 Movement in Strategic (World Map) movement and none in Tactical to emphasize the usefulness of those units that do have increased mobility on the field. 
Love the game, Stardock!  Thank you for the continuing support, fixes, updates and I am really looking forward to Legendary Heroes.  Keep it up and that awesome 85 from PC Gamer will hit Classic 90+ with the expansion.