Hello Nico,
I would play the lower voice as
3, 5, 4 - then immediately take that note over with 5 (i.e. finger substitution)
upper voice :
1, . 2, 3, . 1, 2, 3, 2
(Of course, you don't have to have a legato join between the C and D of the lower voice (a tiny gap actually brings out the long D nicely), in which case the lower voice fingering could be a simpler 3, 5, 5.)
.
If you felt you wanted to have a legato join from the lower voice D down to the next bar G, you could play the last F# of the upper voice with thumb, and immediately substitute 2 (or 3, depending on your hand size) on the lower voice D.
Hope that makes sense!
deW