Dear @k_b ,
My way of doing this may be slightly convoluted, but it works like this:
a) Input a C time signature with a short upbeat (f.ex. C,0.25)
b) Input your “real” time signature - in your case 3/8 - on your first full C-bar
c) Hide the rest in the upbeat bar the usual way (scale and color).
d) Change the barline between the C and the 3/8 time signatures to dashed, and set the dash length in Barline Options (Engrave Mode) to 0. (Assuming that you don’t need dashed barlines elsewhere)
e) You should now have your two time signatures next to each other. Use the Note Spacing Tool to drag them closer together.
f) You will have to repeat (part of) this procedure in the parts as well.
Is there a simpler solution?