I have mixed benzos with DXM twice.
First time was 1.5mg clonazepam with 600mg dxm. The other time was (I think) 2mg etizolam and ~850mg dex.
Want my advice? Don't do it. Both of those times, the amnesia was so severe that I literally don't remember a THING until I started coming down. I remember taking the dex, feeling the first alerts (the warm, opi blanket feel) and then....absolutely nothing, total black hole in my memory. The next thing I remember was coming down, getting typical yet dulled down dex visuals, thinking "huh, I guess I was just floating around the 3rd plat for a while, but I cant remember shit." And then coming back to baseline fairly soon after that.
I also have mixed benzos with ketamine. I am generally against mixing benzos and dissos, maybe a TINY TINY dose of benzo if you are either a) benzo dependent or b) taking a HUGE dose and don't want to freak. When I say tiny, I mean .25mg of alprazolam tiny. But dissociatives by themselves give you amnesia, and if you mix it with another drug that fucks with your memory, you just wont remember a thing and will have in essence wasted both drugs.
As your attorney, I advise you to take the dex first, and save the benzos for once you've come down and want to sleep....or if you're in a little deeper than you want to be, maybe take a tiny bit to chill out.
Oh and as ComfortablyNumb said, mixing CNS depressants is really risky business so unless you're tolerant and know what you're doing, keep the doses as low as possible. You can always dose more, never less, compulsively redosing dissociatives and benzos has led to many a blackout/freakout/bad time.
Also FWIW, I can give a bit of a trip report from the time I took 850. My trip sitter was very concerned with me, because when I finally came out of my blackout and back to reality, I kept asking him the same questions over and over and over. In his own words it was like I had "goldfish memory." I remember that evening, talking with my friend, thinking my words were making sense, but the expression on his face showed me that I wasn't making any goddamn sense at all. "This is probably what being brain damaged feels like," I thought to myself. Since then I haven't mixed dissos and benzos (except for taking the benzos on the comedown), and I also haven't gone above 600mg dex...anything above that and the side effects become too pronounced for me.
Hope that helps
have fun and be safe.
EDIT: Sorry, its early, I guess I didn't read your original post well enough. At first I thought you were going to just combine them. Taking clonazepam wont exactly "end" the trip, it will for sure dull things down a bit but like I said your memory will go kaput. I would suggest no more than .5mg clonaz if you're in too deep and freaking out. And if you need to sleep, .5-1mg should do the trick nicely, but I would wait until you're mostly down from the DXM.