I tried this out of interest, and played according to the tab as follows:
make your G shape as if you were strumming, but only play 1st finger (a G onthe E string) and third finger (D on the B string) together -> let go of G shape, at the same time leaving third finger on slidiing to 5th fret -> then use your first finger to play the 3rd fret (D) and open D double stop at the end of the measure.
This puts your first finger in the right area to move along to the first fret for playing the C part of the next measure (you can be forming your C shape while you are playing the the double stop - ready to place down to start the next measure.
That worked for me - if you can make sense of it!
