My first suggestion would be to check out the Sonlight Forums if you're not already registered there (https://forums.sonlight.com/). Lots of Sonlight veterans frequent there and are happy to share suggestions for making two Core programs work.
Since both students will be studying World History, you may find that it works out to have discussions about the same events or characters. Your older student may find it fun to "listen in" to a Core B Read-Aloud ... so feel free to skip some of the Core G Read-Alouds on occasion. You can always pick them up for extra reading over the summer. Involve your 11 year old in teaching his younger sibling. Let him do some of the reading for you. You may also find some of the Read-Aloud titles on tape at your local library. That would help to save your time and your voice!
In general, I would expect your 11 year old to begin working more independently. Help him to learn to follow a schedule, and initiate his work on his own. Then he can just "check in" with you when you have a break from teaching the younger student.
Basically ... I would spend a few minutes to set your goals/priorities for the coming school year. What are your top two academic and top two spiritual goals for each of your students? Write them out on an index card and tack them to your refrigerator or someplace where you'll see them regularly. When the days come where you feel overwhelmed (and we both know they will come!), take a look at those priorities and give yourself permission to skip everything else that day except for those priorities. Maybe even for a whole week ... especially in January and February!
Hopefully those thoughts, and suggestions that I'm sure you'll find out on the forums, will give you the confidence to move ahead with two Cores this fall.
NOTE: Adjust wording to fit specific situation.