The short answer: upload your video to one platform, then either share it to the other using the built-in share sheet, use a scheduling tool that posts to both, or publish to both at once from an app that connects to each account directly. The third is fastest; the first is free but degrades your video.
Method 1 — the built-in share sheet
Both apps let you share a finished post onward. It costs nothing and needs no extra software.
The catch is quality. When you export a video that a platform has already compressed and then upload it somewhere else, it gets compressed a second time. The result looks noticeably softer than your original file, and on short-form video that difference is visible.
The larger catch is watermarks. A video saved from TikTok carries a TikTok watermark, and reposting watermarked content from a competing app is widely reported to suppress reach. Always start from your original file rather than a download.
Method 2 — a scheduling tool
Schedulers let you write once and queue to several platforms. This works well for photos and text.
Video is where it gets uneven. Some tools cannot publish video directly to every network and instead send a reminder notification at the scheduled time, expecting you to finish the post by hand. Before committing to a tool, check whether it truly publishes video to the platforms you care about, or only reminds you.
Most schedulers are also web-first, which means a laptop, which means exporting footage off the phone you shot it on.
Method 3 — publish to both from your phone
An app that connects directly to your accounts can send one upload to both platforms at once, from your original file, with no re-compression and no watermark.
This is what BeamLoop does. Pick the video from your library, connect TikTok and Instagram once, and publish to both in a single action. The same upload can go to YouTube, Facebook, X, and Threads at the same time.
Write different captions for each
Posting identical text everywhere is the most common mistake. The audiences behave differently: TikTok rewards a hook in the first line, Instagram tolerates longer copy, and hashtag conventions differ on each.
BeamLoop's Creator and Pro plans include per-platform captions, so one upload can carry a different hook per destination without creating the post twice.
Should you post at the same time?
Simultaneous posting is fine and is what most creators do. If you want to test which platform drives more of your growth, staggering by a few hours makes the attribution clearer. Scheduling exists for exactly that.