You can definitely run both guides at the same time with no problem.

I would look more at the issue of your children are less than 2 years apart and so therefore probably could do the same guide. Is it worth it to have two guide going when you could be doing one and beyond that two back to back guides just more for the sake of having the older one in their own space? I am running back to back guides meaning that I am repeating RTR this year having done it last yea5r with an older student. I don't mind it and in fact enjoy it. But I am way over the top all about HOD

and I am teaching it first beefed up at a higher level than intended then the second time at a much lower level so it feel much different than repeating the same guide.
So honestly this is a question you can answer knowing yourself and your children how will that work. You can do both guides together with no problem. The question is would it maybe be worth considering the alternative. And to me the alternative would be combining both either in Bigger or Preparing. With Bigger it would most likely be your youngest doing as written and for your older child the extension pack (thus giving him kind of his own thing). And with Preparing it might be the lower level extension for your younger son and the upper level for your older OR the best fit for your older child either the lower or the upper level extension pack and perhaps no extension for your younger child. Both options giving a different feel for the older and the younger child as your older son wants. You could definitely spin it that way to your son as well.

And if you combine choosing the program that when you take an honest look at particularly where your younger child falls would more likely be the deciding factor because HOD works best when you cater to the younger knowing you can beef up with extensions for the older and have a great fit for both. That would be my choice just having done a lot of HOD and seeing down the road how combining does work so well in general with a 2 year span given one is not terribly behind or extremely ahead of placement by age.
I hope that all makes sense

and I hope it helps. I know others will have helpful advice as well. The ladies here are wonderful about helping you decide and once you do it is clear sailing. Just the decision of where to stat that takes a bit of time and thought.
