If it isn't bothering your two year old, then you shouldn't let it bother you. I put my kids to bed together (they are not quite 2 years apart) up until very recently and I always did exactly like you-let them each pick a book and read to both at the same time. And, the exact same thing would happen-my older one would chit chat, ask questions, and flip back to previous pages. Actually, I can't say it was strictly the older boy-they could both be equally disruptive, or, "inquisitive"-sometimes at the same time.
The solution is to just be patient and relax your expectations of how story time "should" be. Reading doesn't have to be enjoyed by silent passive listeners. It's ok for them to be asking questions-you just want to guide them towards waiting until you pause at the end of a sentence or page and then to refocus back on the story-teach them to be polite if they must interrupt. The extra conversation/explanation is just as beneficial and enriching as the story itself. By allowing them to ask questions and talk about the story you are actually teaching them to be better readers!
That doesn't mean it should be a free for all though-seeing as it's bedtime you need to stay on schedule (can't have them up all night!) The best way to deal with questions, in my experience, is to answer them with a question: "what do you think" is the best one (that's also the best response to the incessant "why's" during that phase of their life, btw.) The second way is to say "let's keep reading and find out." If they go off-topic, I'll say "let's finish the story and then you can tell me all about ___." Then, make sure you actually listen to them after and maybe ask them if they know what made them think of that during the story. My kids never cease to amaze me with the connections they make to the stories or pictures.
On nights where you find that one child isn't interested in one of the books (say, the 4yo isn't interested in a "baby" book the 2 yo picked, or the 2yo can't follow the "big kid" book the 4yo picked) then you might want to have a plan B. I realize it's more convenient and efficient to read to both at the same time, but on occasion it's probably not feasible, especially if you are sure that one kid is ruining it for the other. Pick your battles! I would suggest you let the uninterested kid play quietly in the room while you give the other child one on one story time. I don't keep many toys in my boys' room-it's bad sleep "hygiene" to have distractions like toys or tv's in the bedroom-but I do keep a few things like puzzles, animal figures, and bristle blocks for "quiet time," such as when I am reading to the other child. When finished with one kid's book, switch to the next one (if they want) and whoever just had their turn reading can either have a turn playing (like the other one just had) or can join and be read to twice. Some nights, both my boys would both decide they weren't all that interested in sitting and reading books at all and would rather play quietly. I would read to them anyway while they played. They still get the benefit of the story because their little minds are "listening" all the time.
In summary-don't worry too much about "maximizing" the bedtime story experience for your 2 yo at the expense of your 4yo or, worse, a peaceful bedtime. If you are able to get the books read and both kids to bed without strife by yourself you are doing just fine.