They actually signed a contract to start building such things and cooperate about a year ago

Excellent development, bringing 2 good companies that both are entering rough waters together in a joint effort.

I thought this happened once already, and it failed? Or what that the liscence agreement for J++?

Microsoft had a license to create a JVM according to the standard license for doing so as supplied with JDK 1.0 and 1.1.
This license specifically allowed Microsoft to create their own user interface library (as was normal at the time, but Microsoft were the only ones to actually do it).
When Sun became ready to release Swing (then called JFC) they changed that license to disallow 3rd party user interface libraries and immediately sued Microsoft.
That case hung in court for years and years as Sun had to wiggle into ever tighter corners to get at the money, in the end they got an injunction only on Microsoft having put their own library (WFC) into VJ++ as the default rather than Sun's AWT.

It went on from there until Microsoft got tired of playing with Sun and gave them $2 billion in exchange for a strategic alliance, which is now bearing fruit.

Hmmm...Maybe something good will come out of this.

I bet something like this would be hard to fight in court. I mean, would they have to hand select jurors that knew what the guys were talking about?

Yes, but that doesn't stop lawsuits about highly technical issues.
And with the highly successful anti-Microsoft campaigns being waged in the popular press and on the net it's now quite possible to find a jury to give you the verdict you want if you want to hurt the company (as was shown in the anti-trust trial which was based on quicksand yet got an easy conviction).