Can I post the same video on TikTok and Instagram?

The watermark is the whole issue

A file pulled back out of TikTok carries TikTok's watermark, and the other platforms can see it as well as you can. Removing it is forbidden by TikTok's developer terms, with no exception, even when it is your own video.

So the path is the boring one: keep the file your camera made and send that to each place. It also looks better, because it never went through a second compression.

Same video, different text

What has to change is the caption, not the picture. Every place has its own character limit, its own hashtag ceiling and its own rule about links, and a caption that fits one gets cut where it hurts on another.

Your words get fitted to each place here, and the links are given their room before the prose, so a cut can never carry a tracking parameter away with it.

What never happens to your file

• No crop, no second compression, no vertical squashed into a square.
• No watermark removed, ever, and no route in the product that downloads from TikTok.
• One upload, one clean copy for each place, all of them from your original.

What people actually ask

Will Instagram bury a video that came from TikTok?

Another app's visible watermark is the part you control, and it is the part these platforms have said they push down. Sending your own file takes the question off the table.

Do I have to write a caption for every place?

You write once. Fitting it to each place is the work the tool does, including the character limits and where a link is allowed to go.