The first order of business for a competent Second Crusader would be to placate Roger II of Sicily. He was powerful, could have been a useful ally, and mucked up the edges of the Crusade IOTL by fighting with Byzantium.
How is placating Roger really possible, not sure. He's fought or disagreed with the Papacy, Byzantium, and King Conrad III of Germany for years. Also his feelings toward the Crusader States probably aren't so good as old King Baldwin I of Jerusalem married then threw out Roger's mother Adelaide. And their wedding contract had said if Adelaide bore no offspring for Baldwin, then Roger would become the next King of Jerusalem, which clearly didn't happen.
Some ideas:
- Have Pope Eugene III and King Conrad III (also King of Italy, for what it was worth) confirm all of Roger's gains in southern Italy.
- Promise him that one of his sons will become head of a Crusader State, say a newly conquered Aleppo.
- If he has an eligible/legitimate daughter, get a promise from Baldwin III of Jerusalem to marry her.
- Find him an appropriately tempting royal/noble wife, as he has been a widow since 1135 (and didn't remarry IOTL till 1149).
If Roger can be placating, what would the Crusaders like from him in exchange:
- At a bare minimum, don't go to war with Byzantium and have Byzantium actually believe he won't go to war with them. Perhaps send a son to live as a hostage?
- Extremely helpful would be to have his fleets transport the armies of Louis VII and the other French Lords to the Crusader States. With his past conflicts with King Conrad, ASB to think either side would trust putting German soldiers on Roger's ships.
- Amazing support would be to have Roger II send actual troops to fight in the Crusade.
Possible Benefits:
- The Byzantines aren't distracted by having to defend at land and at sea against Roger II's fleets, raids, and invasions. Theoritically they could focus more on the Crusade (which might not be a good thing) with supplies, ships, and maybe even troops.
- If Louis VII goes by ship, that removes the logistic stress and accompanying bad feelings caused by two large forces, separated by only a couple months, marching across the exact same land route through Hungary and the western Byzantine Empire to just get to Constantinople.
- If Louis VII arrives quicker in either Byzantium or the Crusader States by taking ships, then the campaign season might start sooner.
- A more secure Byzantium, with better feelings toward the Crusaders, might (a big might) be willing to use its fleets to transport King Conrad III(brother-in-law of Emperor Manuel's wife) and the German Crusaders to the Crusader States. Thus avoiding in its entirety the disasterous German march through Asia Minor.