Luckily for you, I made such a list a few weeks ago while research hybrids for a possible all-hybrid campaign =)
The following are what I call "optimal multirole hybrids". They cover two roles, they share at least two recommend stats, and at least one of those stats must be used as a primary attack stat for both classes' at-will powers.
I've listed each possible combination twice, so you can search for a "primary" class first, and then see your "secondary" options. Class order is per current DDI.
(Interesting sidenote: There are no "optimal multirole hybrid" combos for the Rogue, Assassin, and Monk classes. Due to lack of Dex classes outside the striker role. Of course you could make a good hybrid from these, however you'd have to violate one of the assumptions, and even then it would be subpar at best.)
L: Cleric, Warlord, Bard, Shaman, Ardent, Artificer (17/56)
D: Fighter, Paladin, Warden, Battlemind, Swordmage (14/56)
S: Ranger, Warlock, Avenger, Barbarian, Sorcerer (17/56)
C: Wizard, Druid, Invoker, Psion, Seeker (8/56)
The hardest ones to hybrid with are the Controllers and Defenders, so maybe have each player choose one of those unless someone wants double up. This is a good idea anyway because out of all the combos Controller+Defender is the toughest to pull off mechanically. After you figure out that you can go through the class list to see what works for mixing in the leaders & strikers.
Cleric/Fighter (Must be STR Cleric)
Cleric/Paladin (Must be STR Cleric, must be STR Paladin)
Cleric/Ranger (Must be STR Cleric)
Cleric/Barbarian (Must be STR Cleric)
Cleric/Warden (Must be STR Cleric)
Cleric/Seeker (Must be WIS Cleric)
Fighter/Cleric (Must be STR Cleric)
Fighter/Ranger (Must be STR Ranger)
Fighter/Warlord
Fighter/Barbarian
Paladin/Cleric (Must be STR Paladin, must be STR Cleric)
Paladin/Ranger (Must be STR Paladin, must be STR Ranger)
Paladin/Warlord (Must be STR Paladin)
Paladin/Barbarian (Must be STR Paladin)
Paladin/Sorcerer (Must be CHA Paladin)
Ranger/Cleric (Must be STR Ranger, must be STR Cleric)
Ranger/Fighter (Must be STR Ranger)
Ranger/Paladin (Must be STR Ranger, must be STR Paladin)
Ranger/Warden (Must be STR Ranger)
Rogue/Nothing!!!!!!!!!!
Warlock/Bard (Must be CHA Warlock)
Warlock/Ardent (Must be CHA Warlock)
Warlock/Battlemind (Must be CON Warlock)
Warlord/Fighter
Warlord/Paladin (Must be STR Paladin)
Warlord/Barbarian
Wizard/Artificer
Avenger/Druid
Avenger/Invoker
Avenger/Shaman
Avenger/Seeker
Barbarian/Cleric (Must be STR Cleric)
Barbarian/Fighter
Barbarian/Paladin (Must be STR Paladin)
Barbarian/Warlord
Barbarian/Warden
Bard/Warlock (Must be CHA Warlock)
Druid/Avenger
Druid/Shaman
Invoker/Avenger
Invoker/Shaman
Shaman/Avenger
Shaman/Druid
Shaman/Invoker
Sorcerer/Paladin (Must be CHA Paladin)
Warden/Cleric (Must be STR Cleric)
Warden/Ranger (Must be STR Ranger)
Warden/Barbarian
Ardent/Warlock (Must be CHA Warlock)
Artificer/Wizard
Artificer/Psion
Artificer/Swordmage
Assassin/Nothing!!!!!!!!
Battlemind/Warlock (Must be CON Warlock)
Monk/Nothing!!!!!!!!
Psion/Artificer
Seeker/Cleric (Must be WIS Cleric)
Seeker/Avenger
Swordmage/Artificer
Keep in mind that some of these are still iffy due to mechanics other than stats and roles. Avenger/Seeker & Cleric/Seeker are examples. Good combo at first sight...however the ranged weapon nature of seeker conflicts with melee weapon nature of the Avenger and Cleric. Which means it would probably turn out crappy. Same goes for mixing implement with weapon users, and heavy area attacks classes with melee weapon attack classes. Those can all be done, they just turn out a bit less optimal.
