Weighted soft hackles

[Mataura mayfly]

I always use the weighted fly as "first in line" or front fly.
Because...... I normally always tie the lighter soft hackle off the bend of the front hook and do not tie it on a separate dropper in the leader system. If the light fly is the tail or point fly, to me it assumes a more natural drift than if it were constrained between the taunt leader and weighted end fly.
However, if you decide to fish the lighter fly on a dropper attached to the leader and had the weighted fly on the terminal end of the leader, the action of the lighter fly would be more natural and yet it is being presented at a level of depth to target your quarry. Personally I try to avoid droppers as they lead to some interesting twists and tangles (in my experience), but there are folk out there that swear by the system and are more than likely much better casters/fishers than I am ever likely to be.
Another reason I like the heavier fly to be in front is as a visual aid. Often my front weighted fly will be a bit flashy and be advertising "hey-look at me", I know it is a pattern not likely to be taken by the local wild Brown Trout population, but it catches their attention and sometimes you can see them follow the "glitz and glamour" all the way through the drift and take the point fly on the swing.

Once again, just my personal observations, others may have better ideas and different suggestions that might work better for you, but I say play around with your weighted fly in both front or back positions and see which works out best for you.