When using the split grade mode you must change the filters as Jerold explained.

The hard channel (channel 2) is not affecting the soft channel (channel 1)
That means that you can fine tune your contrast by the exposing time in channel 2.

When the contrast is decided by the time in channel 1 and 2, you can make your photo darker or lighter by changing the exposing in channel 1. Channel 1 is affecting the hard channel. The channel 2 will change and keep the contrast as you have decided.

So you should decide the soft expose first, then the hard expose.
(Sorry for my english)