The designated marksman is carrying a DMR along with an AR.
Standard practice?
Also, wouldn't it be better if he carried weapons with same calibre and not a 7.62 and 5.56 composite combo of guns?
If you're talking about the guy on the far left, that's an AK he's got, not an SVD.
If we assume the weapon belongs to the guy in the red jacket, then I suppose the guy above is holding it for him as he gets down from the truck:
It does have an awful long barrel though, I can see why someone would think its a Dragunov. Weird, AK barrels aren't usually that long...unless this is some Eastern European jugaad variant captured from hajis.
Why would they carry two assault rifles? Well...setting any personal assertion of the soldiers aside, I'd say if you got pissed off and wanted to maim a jihadi or two, doing so with an AK would be far more arbitrary from an evidence standpoint, considering everyone uses AKs whereas a 5.56 weapon would draw the focus onto special units considering INSAS is rare in CI/CT (not that jihadis don't get hands on few AR-15s here and there, smuggled from Afg). Just a hypothesis.
Also, there is the tactical explanation. Got enemies behind hard cover like stonewalls? AK will shread them up nicely in closer ranges...5.56 not so much. So makes sense for 1 or 2 guys in a team carrying one as a means of support. It's very unconventional, must admit, but far more unconventional things have been known to happen among SOF during times of war, even American counterparts.
I'm inclined to think a combination of both factors.
To answer your actual question (though it doesn't apply here as this isn't a DMR), yes, it was pretty standard practice for a sniper/DM to carry another primary weapon (like an M4 carbine) in US military, especially SOCOM. Not unlikely to see that replicated here.
Though this necessity became mostly non-existent later on with the adoption of AR10 or AR15-based precision rifles typically used by marksmen in urban environments with roles mirroring that of our CI/CT. With platforms like those (especially with an LPVO thrown in), one can run the same rifle for everything from CQB (not for assaulters, but for clearing your way to a sniper perch or something like that) to long-range shooting.
We don't have that luxury with our current inventory though.