How do you structure ucas personal statements (physics or engineering)

Tbh I would wait to plan your personal statement later and use this time to find out which degree is for you first so it makes it easier to plan it later on.

when applying what if I apply at one university for engineering and physics at another

Why??? I’m not bashing you or anything I’m just a bit confused.

In that case you'll want to have a personal statement that satisfies both of those courses. The courses are similar, so this is possible, however it could lead to a slightly mismatched personal statement if you end up with an engineering section and a physics section. In many cases such a personal statement can still work fine, however if you are applying for top universities (e.g. Oxbridge) it may be to your detriment.

However, to second what's been said, I think you may benefit from taking a bit of time to decide on one or the other. Ultimately, by applying to both you're just deferring the decision for which you prefer until later! Maybe look at virtual open days to decide which specific course content appeals to you, or do some wider research. Deciding on one or the other might take some thinking but you still have ample time.

You may find my tips on writing an engineering personal statement helpful regarding how to structure your statement. Even if you go down the Physics route, it may still be beneficial. Please note it's quite Oxford oriented but still useful, I hope! https://www.thestudentroom.co.uk/showthread.php?t=6459234

i want to do a physics degree then do an engineering degree then become and engineer then later in life work in education as a physics teacher

Hello! Even I’m stuck on choosing between the two, you said you’ve been reading some books on physics? What are some of those books so I can try read them too?