If I am teaching two children who are the same age, would I need to purchase 2 copies of some of the books?

Only if they don't share well.

If you schedule their reading times for the books at different times and they are not workbooks (or books that one of your children will highlight or mark in some way special to him), I can't see why you'd need to purchase more than one copy of a book.

That makes sense. Thanks Ray!

I have a 9 year old daughter and an 11 year old son that did UG together last year. Now, keep in mind that they would really not go out of their way to help each other out. There were very few times that they both wanted or needed to read the same book at the same time.
Maybe we are different from many families, but my children have a schedule of all work (from all subjects) that they need to do in a given day. If my son was reading the book that my daughter wanted, she just needed to work on another subject until it was available.
But the one thing I did to help avoid this situation was to schedule the readings so they would not be on the same day. Usually there is a history core reading, maybe a church history or Bible reading, then possibly a literature reading. I would schedule my son to do history reading on Monday, literature reading on Tuesday, then Bible reading on Wed. I would schedule my daughter different readings on different days. It's very doable! There are probably other ways to go about it also.

I had 3 doing UG this year at the same time. I go over the next week's schedule on the previous Friday (in case anyone is extremely motivated and wants to read on the weekend). Then they have Monday through Wednesday to get ALL their reading done. There are usually at least 4 books to be reading from, so with plenty of time and several books...it worked out. The only times they wanted the same book was when they REALLY liked it and all wanted to read the same one first!

That's very helpful. It definitely sounds very doable. Thanks so much for sharing how you manage it. Much appreciated.

This has been a question for me too. I am thinking though for literature in the D and R levels it might be helpful to each have a copy if possible for the benefit of refering to specifics when discussing. I would like to have my copy too. But this is way too expensive. How do you all handle times like this?