Yes, you can! When you've started writing a comment on a thread, but want to continue it later, you can save it as a draft quite easily.
After you've started writing the comment, simply go to another part of Twist. When you return to that thread, your half-written comment will be waiting for you to finish it.
Drafts are only visible on the platform that they were created on. For instance, if you save a draft on the web, you will only be able to see it on the web. You won't be able to see it from the Android app.