when i go dual heroes from the start, i'll build the tower first, so the first champ is unlocked after 8 turns or something. my favorite champs for dual hero teams are defenders, so if i get a chance to pick hannis or inslow (play mostly kingdoms, don't really know the empire heroes by name) i'll get them and turn them into a tank ASAP. other good choices that i used successfully are iriel (starts with life 2 -> useful as a healer from the moment you pick him up) or badra as assassin (got lucky with her - found a toxic shortbow in a blue treasure chest, made her pretty deadly right from the start). depending on what champ i picked up, the sov will usually pick a path that complements the champ. can be anything, really, as long as you can somehow turn them into a into a proper team with some synergy (commader + warrior, or tank & healer, or tank & assassin, whatever).
in my experience over the last 5-6 games, an early defender champ is really awesome. they actually don't need long to become very tough. at level 4, they have defensive 1&2, which puts them at a quite respectable 30 defense naked (obviously more if you pcik up/buy some cheap leather items). even without gear, a defender champ can tank 3-4 groups of swarming mites, darklings, some wolves, bandits etc. use them wisely (retreat unarmored troops on turn 1, let the tank get the enemies' attention, then move forward with the spearmen/militia and flank them, kill them one by one; the other champ can heal, slow, do damage, command or whatever).
maybe i'm just lucky, but in my experience, if you push hard and try to do tough encounters early, your champs will level up reasonably fast despite the split, and grab some decent items that would otherwise go to waste. it's just great to pick up a "warding kite shield" from some low quest that will skyrocket your defender's efficiency, or clear that corpse spider lair on turn 25 and find a razor sword that turns your level 5 warrior into a beast that can onehsot strong single mobs with >100 crits, picking up that stuff later isn't all that impressive, but a nice early item drop can turn one of your guys into a real powerhouse 50 (or more) turns before your troops can get even close to what he can already do.
in my current game (huge/epic/expert/dense monsters/quests/wildlands) i never split up my dreamteam (healer sov and tank champ) and they did some quite impressive stuff together, such as killing a giant rock spider guarded by 3 rock spiders and 4 packs of spiderlings, or stumbling into that dreaded umberdroth & stone golem lair in the pits of namtur and killing 3 umberdroths and half a dozen stone golems (unfortunately only one of their 3 spearmen units survived that fight), or killing several stacks of 3 crag spawns & 3 earth shrills before turn 50, or routinely killing "deadly" death demon/fire lord/earth elemental lairs with nothing but spearmen as backup, or wiping out the cindercorpse lair from the skeleton event. that game is on epic pace, so obviously you'd have better troops much sooner on standard tech pace, but in my book, those two heroes aren't underpowered or useless - they are basically the driving force behind my whole faction. by now, the game is pretty much won, it's turn 180 or something, they are both ~ level 15 i think and can kill pretty much everything on the map, when my best available weapon techs right now are daggers and axes.
when i picked up LH and played the early versions of it (0.50) heroes sucked, because their level progress was really painfully slow. after 2 flat XP bumps and the recent change of bonus XP for harder encounters, i find them very solid. not saying other strategies aren't viable, but my heroes feel very legendary when i play a hero based game. i *probably* could have won that same game without a second hero, but it would have been a lot slower and more painful (i.e. replace dead spearmen after every fight).