Struggling to find answers about sleeping arrangements and strollers

Hey everyone,

We recently found out we're happily expecting a 2nd baby in october, (I know we're early with this thay's why we're posting anonymously because no one knows yet) while we already have a 10 month old as of now.

When the new baby arrives, our kid will be a year and a half (18 months). We were thinking about our sleep arrangements, like a bunkbed for the oldest (obviously in the bottom part) so we have the babybed ready for the newborn. When the newborn gets old enough she can move to the bottom bunk and the eldest can move to the top bunk.

Is this a realistic view? Or will our eldest be too young to sleep in a bottom bunk bed when she's a year and a half or 3 when she moves to a top bunk.

And what are your thoughts on strollers? Is it worth getting a twin stroller or will the 18 month old be old enough to get on like an extension pad at the back for her to sit/stand on. Our concerns here are that if we go somewhere with just the stroller for the baby, our 18 month old won't have anywhere to rest/sleep when she gets tired.

Thanks in advance