I get asked this question a lot. It usually sounds something like this "How can my children who are 2-3 years apart in age actually both learn from the same books?" Usually the older student is brighter, more studious, etc. and they don't want to hold back the older, yet they don't want to teach two totally different levels of each subject to their children.
It comes down to skill-based vs topic-based learning.
Still based are things like math and learning to read. You have to learn the sound of the letters before you can blend them into words. You have to be able to blend easy words before you can read multi-syllable words, you have to know how to multiply before you can divide. You have to understand arithmetic before you can do Algebra.
Skill based subjects have to be built on, precept upon precept on a regular basis to gain mastery. It takes time and you can't teach a child who cannot subtract with one who divides with ease at the same time with the same materials. They need to develop certain skills before they can go on to the next level.
But, then there are topic based subjects. I can be 8 or 10 or 15, but if I have never learned about whales I can learn from and enjoy a book about whales or a documentary about whales. A 15 year old might understand more about the anatomy and ecosystems of the whale when the book is done, but the 8 year old will have learned a lot too--and they can discuss it and reinforce what they learned through discussion.
This is why you can teach varying ages together with Sonlight [or any literature based curriculum] together for topic based subjects like history, science, geography, Bible and cultural literacy. It is also why you will want to teach them separately for math and adapt their writing assignments to their abilities. You will not be holding back the older by teaching them together for topic based studies. The older child can dig deeper and tell the younger child(ren) what they have discovered. They can go deeper in the subject by themselves if they want to, but even if they don't they will have learned something they didn't know and will benefit from it.
And, when siblings learn together it strengthens family culture and bonds. It gives them a commonality, inside expressions, jokes and sayings and is just plain fun. If you have children up to 3 years or so apart, try teaching them at least one subject together and see how it goes. I think you will love it!