You can do that in imgflip with a multi-step creation process. Make a gif with one caption where and when you want it. Copy the web address from that gif back into the gif generator and you can add another caption in a different place and time.
If I use that technique, I delete the first unused iteration(s) to keep the
imgflip.com/creations part of my profile cleaner and also to save server space. Hope that helps.
