Can I use 2 identical rubber ducky antennas with a splitter on my CB HT radio to improve range?
Theoretically yes, if you carefully control the lengths of the cables and the placement of the antennas. This is called a phased array.
However, you would get a far better improvement in range by using an antenna which is not a rubber ducky, like a 1/4 wave whip or a dipole. Rubber ducky antennas are electrically shortened, making them smaller. The price of that reduction in size is reduced efficiency.