Here is what I am doing vs stacking VSS's. Plus it gives you control for tire sizes and speed.
The SGI-5E can split speed signal to multiple outputs, and, if need be, alter the signal for transmission, tire, or gear ratio changes.
You can feed the VSS direct to the SGI-5E, and use two different outputs to feed different devices. In your case OUT1 to the EFI and OUT2 to our dash.
Each output would have the same signal frequency, but are independent of each other, so the signal strength remains.
http://www.dakotadigital.com/index....ct_id=126/category_id=-1/mode=prod/prd126.htm