Did you rescan channels with both tuners connected?
I suggest deleting Media Center data by following this guide http://www.hack7mc.com/2009/09/clear...ia-center.html
then rescanning. During setup, Media Center should say 'detected 2 tuners'.
After setup, go to guide and right click a channel and click Edit Channel-Edit Sources.
I have 2 dual tuners see attached jpeg showing 4 tuners. Yours should show 2 and they should both be 100% signal.
A splitter will halve the signal to each device. Personally I use a cheap distribution amplifier with 4 outputs to feed my various tuners/pcs.