Step 1: Pick 2 hero decks and 1 variant for each of those heroes
Step 2: Choose one of those character cards to use the base power of and the other to use the incap abilities of. Your maximum health is the average of the two cards rounded down
Step 3: Take at least 1 copy of every different card from each of those decks taking exactly 20 cards from each hero deck
Originally I used these to make a more "gun-y" Guise by combining him with Expatriette. Guispatriette played surprisingly well against Baron Blade, he was less able to play off of Legacy and Wraith's cards but was more able to deal reliable damage and retrieve guns and ammo from his deck and trash. I haven't had much time to experiment with different characters and villans, but I'd be interested in any results anyone else had using these rules.
I long while ago there were some forum games that mixed hero decks. I think they allowed for more than 2 heroes and didn't require to have at least 1 copy of each card.
My biggest recollection of it though was any deck with Mister Fixer was insanely powerful.
All I gotta say for this is
Legacy: God of the Sun.
Ronway: You're probably thinking of Be The Gate. It didn't get as many decks as it deserved; I really liked the idea, but it was work.
On-topic: This will, sadly, generally give you some pretty weak decks most of the time.
As a rule, hero decks only have about 14-18 different types of cards in them. This allows you some measure of control over what you draw, and lets combos be set up. With your system, requiring 20 cards from each hero, and also requiring every card possibility from each hero, you'll be looking at anywhere from 28-36 unique cards to choose from in their deck.
There are a couple of combinations that might be pretty beefy. Wraith + Omnitron should work, I think. Naturalist + an Ongoing-heavy person could be interesting. Absolute Zero + Ra, very potential edge case.
Immediate follow-up! I was curious, so I sat down to see what Upgrade (The Wraith + Omnitron-X) would look like. This card uses O-X's Power and Rook City Wraith's Incap.
HP: 26
Power: Reveal the top card of a deck. Put it into play or into the trash.
Incapacitated Power: Choose one each turn
Select an Equipment card in a Hero trash. Put it into Play.
Select a Hero. That Hero regains 2 HP.
Destroy an Environment card.
One-Shot
Grappling Hook (x2)
Impromptu Invention (x2)
Inventory Barrage (x2)
Suture Self (x1)
Throat Jab (x1)
Trust Fund (x1)
Disruptive Flechettes (x1)
Reset (x1)
Self Sabotage (x1)
Singularity (x1)
Technological Advancement (x2)
Ongoing
Bio-Engineering Beam (x1)
Ongoing, Limited
Combat Stance (x1)
Defensive Blast (x1)
Reactive Plating Subroute (x2)
Slip Through Time (x1)
Equipment
Infrared Eyepiece (x1)
Mega Computer (x1)
Stun Bolt (x1)
Rocket Punch (x1)
Equipment, Limited
Micro Targeting Computer (x2)
Razor Ordinance (x1)
Smoke Bombs (x1)
Throwing Knives (x1)
Utility Belt (x2)
Equipment, Component
Electro-Deployment Unit (x2)
Focused Plasma Cannon (x1)
Gaussian Coil Blaster (x1)
Innervation Ray (x1)
Equipment, Plating
Ablative Coating (x1)
Elemental Exochassis (x1)
Temporal Shielding (x1)
More likely he's thinking of the four (more?) Sentinels Deckbuilding games we played a couple years before Be The Gate.
They were lots of fun and stupidly broken. I don't remember if it was one of these games or another where we managed to play Wrathful Retribution four or five times in one round. It was bananas.
Edit: Definitely more...
Edit 2: Nope, the Wrathful Retribution x4 was with normal decks in Waiting for Capt Cosmos
I haven't had much of a chance to thoroughly test one of these decks, but when I do build these decks I prioritise card draw and deck/trash search. I've found that the less reliable combo-building is offset by a greater variety of powers and card-cycles and abilities unavailable to either base hero.
My current favourite is a combination of Parse and FFWraith on the incap.
HP: 27
Power: Deal 1 target 2 projectile damage
Incapacitated power: Choose one each turn
One-Shot
Trust Fund (x2)
Between the Lines (x1)
Syntactic Analysis (x1)
Impromptu Invention (x2)
Targeting Arrow (x2)
Throat Jab (x1)
Recompile (x2)
Grappling Hook (x1)
Data-Mining (x1)
Quick Calculation (x2)
Suture Self (x1)
Impossible Shot (x2)
Inventory Barrage (x1)
Ongoing
Segmentation Fault (x1)
Exploit Vulnerability (x1)
Extrasensory Awareness (x1)
Critical Multiplier (x1)
Buffer Overflow (x1)
Reveal the Flaws (x1)
Gauge (x1)
Ongoing, Limited
Updated Intel (x1)
Combat Stance (x1)
Snap decision (x1)
Equipment
Mega Computer (x1)
Infrared Eyepiece (x1)
Stun Bolt (x1)
Equipment, Limited
Smoke Bombs (x1)
Razor Ordnance (x2)
Throwing Knives (x1)
Micro Targeting Computer (x2)
Utility Belt (x2)
I like the way "Trust Fund" gets some extra utility from the discards, "Micro Targeting Computing" buffs most of Parse's one-shots. Her incapacitated side also helps other combined heroes by allowing them to search for specific equipment.