Both fans would have a tacho signal. The problem is that some fans give 4 pulses per revolution and others will give 2. You also need to take into account that different fans will spin at different speeds so the HP firmware might be freaking out because the stock fan would never go that slow or fast.

The Papst fans I've got do have a speed signal but it's slower than what most things expect. My crappy Shuttle P3 board BIOS would freak out because the fans appeared to be turning too slowly.