I got this from Zap2It's TV Gal, so you know it's the goods:
"There's always a crossover episode during sweeps. This time it is between 'Third Watch' and 'Medical Investigations' on Friday, Feb. 18."
This brings 'Medical Investigations' into the blend with 'ER' as they did a crossover with 'Third Watch' a few years back.
There are plenty of other connections to be made, due to in-jokes and appearances by people as themselves, but this is really the only major, official, one.