[PyMOL] SS in a trajectory

Hi,

I have a trajectory file, in which two short helices would merge into one long helix.  However, when I play the trajectory in Pymol as cartoon, the long helix would still be shown as two helices with a loop in between.  Is there a way to force Pymol to show a long helix for a specific range of frames in a trajectory? Thanks a lot.

--Jianghai