Here is another way. Start with the original clip in the Collections window
and then click on it so that it is displayed in the Preview window. Preview
the clip and pause it where you want to make a split. You can fine tune the
frame to make the split by advancing and rewinding the clip using the arrow
keys on Preview window. When you are ready to make a split, click the Split
button on the Preview window. You will that see the two clips in the
Collections window. To recombine these two clips highlight them both in the
Collections window. Then on the WMM toolbar click Clip/Combine (Ctrl +M).
This only work for two clips that are contiguous.