Well, I'm no expert on published adventures but I've never heard of something like that. My suggestion would be to use the gestalt rules presented in Unearthed Arcana and just run a 4 player adventure.
The only downfall of using Gestalt characters is that the characters themselves are about 1.5x as powerful as a single-classed character of their level however they are still limited by the number of actions they can take in a round.
I'd recommend having a look at the following thread and running an adventure designed for 4-players based off of some of the suggestions contained therein:
DMing For Smaller Groups, Picking Up After A Player Leaves
My advice: Give each of the 2 players an equal level cohort in a complimentary support class (i.e. bard, cleric). The players control them in combat but they do NOT actually roleplay that character since 1 player RPing 2 characters is generally a disaster in the making.
You could also just run an adventure / campaign designed for 4 players, but at two levels lower than the party.
Extrapolating from CR (I know CR and ECL are different, but it is still a fair indication), A Party of 4 Level 5 characters is an EL of 9. To get an El of 9 with only 2 characters, they need to be level 7. This at least indicates, that 2 ECL 7 characters should face the same level of challenges as 4 ECL 5 characters.
The only thing to keep in mind is if you lose 1 character, there goes half your combat ability, as opposed to only a quarter in a bigger group. So I would suggest less save or die and massive damage enemies, and ones that are more defensive in nature.
Of course, depending on what they choose to play, you could also use the opportunity to run and entirely different style of campaign. EG if they both play stealthy characters, it can be about infiltration, if they both play wilderness characters (Druid & Ranger say), you can have an "outdoorsy" campaign, if they both roll up diplomats (Bard / Paladin) you can have a court intrigue campaign. There are lots of options here.
I would suggest giving them a few more skillpoints so they can cover their bases, and also maybe full hitpoints, and a higher stat generation method (or save bonus), to give them a bit more resiliancy and staying power.