While some S Pen styluses are designed to work with multiple devices, that isn't the case with the Galaxy S24 Ultra's S Pen and the Galaxy S25 Ultra. Not only will it not work, it poses the risk of damaging your device or the S Pen.https://www.notebookcheck.net/FYI-The-Galaxy-S24-Ultra-s-S-Pen-won-t-work-with-the-Galaxy-S25-Ultra.953062.0.html
Do some more research. You can buy the pen from Samsung also from Amazon with Bluetooth and all the actions
I received my S25 on Saturday, February 1st. The S pens frome the S23 and the S25 can be used on both phones (S23 and S25) for writing notes on screen, though I have not tried using them for pictures or syncing them to the other one yet. There is, however, an option in Settings on the S25 where you can select to sync your phone to multiple S pens at once.