The similarities are that both Septimus and Clarissa feel that they are outside floating in the world, looking on at themselves at the world ahead of them and their actions but at the same time dashing headlong throughout the passage of time. They feel very lonely and also very isolated from the world and feel very uneasy about themselves.
But they do differ from each other. Septimus went to war and tried to defend his country from being attacked and attempted to become a "man"" but unsuccessfully lost the war. At the other hand, Clarissa, did not fight in the war. she married a safe man who would not dared her to become more of a woman that she believed herself capable to do it, but she was wrong.
I believe Septimus and Clarissa both start from the same point. They couldn't accept their reality for what it is. But they are, however, differentiated by their choices. Clarissa moves in her life by choosing