The only way would be to route the rpm cable to another header individually, that said i just wanted to test connecting an external 12V PSU to a fan and wanted to see the rpms the fan was going so decided to use a splitter with the rpm going to the a motherboard, but i never got a reading on it, not sure if it need the power/ground also to be connected for it to be signaling the mobo to read.
That's what I'm in doubt of as well. Is connecting the RPM signal alone to the header in question sufficient. Sounds like it maybe isn't. Some guy reported success getting an RPM reading from a fan connected to a PWM splitter in another thread:viewtopic.php?p=522782#p522782
Perhaps you're right, in that you need to connect the fan in question to the header's power/ground and sense/RPM pin, and then only route the PWM signal to the fan. That worked for him at least.
If this is the case, then the attached diagram for a Y "RPM splitter" should work for this, as far as I can tell.
You buy a PWM splitter, and for each case fan that you want a RPM reading for, you connect the fan to this RPM splitter. You connect the 4-pin connector with PWR+RPM+GND to the motherboard header that you want the RPM reading to show up on, and connect the PWM part to one of the female 4-pin connectors on the PWM splitter. As far as I can figure out this should work.EDIT:
Updated the diagram. The 4-pin connector that you connect to the fan is male, and the two connectors you connect to the motherboard and PWM splitter, respectively, are female.