IMHO, it is VERY normal especially with two siblings close in age. My two DSs are two years apart, and we have seen/see it a lot. They're almost 6 and 8 now and it still is an issue although we can now explain to DS8 that DS6 needs to learn, and he understands, although it takes great effort on his part to keep his answers in. I recall setting boundaries when they were as young as yours, where I'd explain to older DS that this was time I was going to spend alone with younger DS. I used a timer, and had older DS do something enjoyable on his own in another room. I tried to reserve something that older DS found "special" for that time. It served more than one purpose, because it helped older DS learn to be able to occupy himself without my attention.
We do still see that DS8 wants the attention, and easily can steal it away from DS6 in many situations. For example, last night DC were talking with their grandparents on a Skype call. DS6 has his very first spelling test today and he was given 5 "challenge words" that were impressive for most 6 year olds and he wanted to share that. DS8, not to be outdone, went and got his challenge list which of course was much more impressive to the grandparents. It is something we continually work on with the kids -- to bolster DS6's confidence and to explain to DS8 that it's ok for someone else to be in the spotlight once in a while.
DS6 has issues with both perfectionism and confidence -- it's very hard to live up to his DYS brother who has 2 years more experience at everything. This doesn't really answer your post, but it may be something you look to in the future. We actively look for things that DS6 is passionate about that DS8 is not so passionate about -- to give DS6 a real chance to shine. It's been hard because DS6 usually goes along with DS8's interests, but we're still trying to find HIS thing, if you know what I mean.
Best of luck! Your DC are lucky you're aware of this sibling dynamic so early. Keep us posted when you find a solution.
