We did this. Things we found out were:
Toddlers and teenagers, not that different 😳
There are few places to take them that they'll both like - big pools with slides, Centerparcs and trampoline parks maybe.
Our younger ones now have a fantastic older sibling to stay with and have sleepovers! Oldest DC can be a useful double agent too!
Oldest one found it harder to get used to a new baby but was much more rational than say a toddler would be.
Older one can watch while you nip to loo etc and have own quite special relationship too.
I'd let older one be as hands on with younger one as possible, I wish I'd done that more.
When you have a teenager going through all the crazy teenager stuff... stuff that you'd never expect and you have a toddler needing soooo much time and attention, make sure you have time set aside for just you and your teenager. It's so important. We had a weekend in London, shopping trips, spa days etc. massively important.
Your eldest seeing all the gross baby stuff gives them a great insight into parenting apparently! 😂
It's not easy but now it's so lovely to have another adult that loves the younger ones so fiercely and protectively.
Good luck!