Hi,
I am no expert but will tell you what has been working for us.
We use English for the Thoughtful Child, half of the book in grade 2 and half in grade 3. This enables us to go through slowly and review often. It takes very little time in a day, and we find the method enjoyable.
English for the Thoughtful Child 2 in grades 4 and 5, similar to above.
Junior Analytical Grammar for grade 6, and we will be using Analytical Grammar this year for grade 7 (and after).
For Spelling Wisdom, we try do dictation twice weekly. We look at the exercise daily and identify any problem words. After the student is confident with them, we do the dictation. I say it and the student writes it out. Then we go over it and correct any mistakes (spelling and punctuation). If there is a misspelled word, I usually have them write it out correctly a few times, copying it to make sure it is correct. That is it.
With the younger kids, I usually cover one lesson a week in English for the Thoughtful Child, and as stated, two dictations. Of course you should modify this to fit the child, but I do not think this is beyond reasonable expectations.
I have all of the kids read aloud every day (sometimes the younger ones will read to an older sibling) and they are read to every day from a good literature selection and also the bible at a minimum.
Just as a sidenote, we have eight children. Their ages are 12, 11, 9, 6, 5, 4, 2, 6 months. Even in a busy house such as ours, I do not find this to be too much to handle, nor do I find it too light to be useful.
I am not sure how you could teach them together. Language Arts is one thing we approach at different levels, because of the different skills they have at progressive ages.
I hope this is a little helpful.
Gaeleen