I looked at the dual channel tach a few years ago as I wanted to log engine & driveshaft rpm. I also found a true data logger that sold for $550 and it will accept up to 12 channels of signals. To me it was a no brainer and I did the data logger.
Are true data loggers legal in Stock and S/S?
The classes I run the data loggers are completely legal.
http://www.rpmperf.com
Actually the system I have "Basic" is now $588. Purchased mine some 3 or 4 years ago.