There is a TON of curriculum out there that is non-graded. If you stick with a lot of that stuff, then you could even get away with telling DH (if he ends up being a hard-nose about it) that yes, kid A is grade 2 and kid B is grade 3, but look! This curriculum is for "grades 1 to 3" so they can do it together!
I'll just echo what others have said. Subjects that are very skills-based like math, sure do them separate. (Though there are options there too, things like Math on the Level which is real-life based rather than workbooky). But a lot of schooling stuff is KNOWLEDGE rather than SKILL DEVELOPMENT. What does it matter if they learn about Ancient Egypt when they're 7, or 8, or 9?
If you've got them in 2 separate grades, using a boxed science curriculum. Perhaps one year you're teaching kid A about simple machines and kid B about gravity. The next year, kid A is learning about plant life and kid B is learning simple machines. Next year, kid A is learning about weather and kid B is learning plant life. And so on.
Won't you get sick of teaching the same thing twice, AND having to separate your two kids for two separate science lessons each day? How much of YOUR time is that wasting?
Wouldn't it be easier for YOU to do them both together, gravity one year, simple machines the next, plant life the next... Easier in that you won't get bored of the material AND it takes HALF as much of your time. And won't it be more fun for THEM getting to bounce ideas off each other, build projects together, do experiments together.
What if you're doing separate grades, and once kid is doing a cool science experiment, and the other has to just sit and watch because it's not on their curriculum? See? More fun to do it together.

And like I said, there is a TON of curriculum out there that is designed for a RANGE of ages and grade levels rather than a specific individual one. Most homeschool curricula makers recognize this natural range of abilities.

You can even tailor things even further... If one is a strong writer and can write a whole page report with one hand behind her back, and the other cries just at the thought of holding a pencil, then they can both get the same lesson on simple machines, but the one can write a report about it as her response, and the other can build some simple machines out of lego as his response.
