Same shape of video, different audience
A vertical video can be posted to both apps. The people who see it are not a shared pool. A TikTok view does not become a YouTube subscriber, and a Short does not add a TikTok follower. Treat each upload as its own post with its own first line.
Shorts live on a YouTube channel that can also hold longer videos. That is the practical difference: a Short can point at a long video on the same channel. TikTok cannot hand that viewer to YouTube.
What is safe to reuse
The footage can be the same file if you have the right to post it in both places. Rewrite the on-screen text and the caption so each one makes sense if the viewer has never seen you. A watermark from the other app is a reason for someone to feel they are in the wrong place. Remove it.
The first second still has to match the rest of the video. If the opening promises a result, show that result. Do not assume a sound that is popular on one app is available or useful on the other.
What does not transfer
Posting time, hashtags, and a "best" hour from one app are not instructions for the other. Use the Shorts posting grid and the TikTok posting grid as separate starting points, then replace them with your own results. Neither tool posts for you.
A view count on one app is not a prediction for the other. If a TikTok did well, you can still post the Short. Judge the Short on its own YouTube numbers.
Where to spend the week
If your longer videos are the thing you want people to find later, put the week into Shorts that point at those videos, and use TikTok only when you can write a native caption. Plan both in the content calendar. Paid Creator Climb plans audit the YouTube channel. They do not publish to TikTok or Shorts.
Questions
Should I post the same clip to both?
You can, if you have the rights and you rewrite the caption and cover text for each app. Remove the other app's watermark. Judge each post on its own numbers.
Do TikTok views help a YouTube channel?
Only if a person actually comes to YouTube. The apps do not share subscribers. A Short on your channel can point to a longer video. A TikTok cannot do that for you.
Is one app better for growth?
Neither is universally better. Pick the one where you can keep posting and where the viewer you want already is. This site does not have a chart that settles it.