Based on my game yesterday, I'd say two leaders in a party of four works just fine.
My Battle Cleric and the Resourceful Warlord can both act as front-line, Combat Advantage-granting combatants because we know that we will pop back up quickly even if our relatively low AC values cause us to take extra hits and go down quickly.
Extra healing is extra HP that can be doled out to whoever needs them. Also, being front-line means we absorb attacks that otherwise would focus on just one or two defenders or strikers, unlike my earlier Laser Cleric who went through numerous battles without a scratch.
Here is a 4 character party that I think would work very well, using 2 leaders and multiclassing to get some Controller powers and extra damage:
Tactical Warlord with MC Wizard
Battle Cleric with MC Ranger or Invoker (depending on ability scores)
Shielding Swordmage with MC Wizard
Brutal Scoundrel Rogue with MC Ranger (extra damage and free skill)
All 4 characters go into melee, thus there is no need for a Fighter to lock down monsters and prevent a breakout into the squishies. The Swordmage can reduce damage to whoever gets hit. The Rogue will almost always have Combat Advantage on something, if not necessarily the BBEG. The two leaders provide lots of healing to keep everyone on their feet.
Lastly, using Readied Actions, the group can cooperate to use the Cleric's Righteous Brand bonus against the biggest threat for multiple attacks. In this group, I can see up to 6 attacks with the bonus, ignoring the possibility of OAs. Assuming the Swordmage gets the Righteous Brand bonus:
First round:
Setup: Cleric attacks with Righteous Brand (if it misses, just try the sequence again next round)
1. Swordmage readied attack triggered by the end of the Cleric's attack.
2. Warlord's readied action uses Commander's Strike to have Swordmage attack.
Second round:
3. Swordmage attack
4. Swordmage uses Action Point for additional attack
5. Warlord uses Commander's Strike to have Swordmage attack.
6. Warlord uses Action Point for addtional Commander's Strike.
Epilogue: Cleric goes again, and hopefully sets up the Righteous Brand trick again, but without the added zing of the Action Point actions.
Maybe that's too munchkin-y for some people -- my own group didn't go for a variant of it -- but as far as I can see it's there in the rules, and probably already on the CharOp boards.