The more compatible the outlines are in advance, the better.
In fact the Blend tool gives the most reliable results, because it may add nodes to the original outlines.
The point of MM is that the node structure of each master is identical. So deleting a node in one master means deleting it in the other master too. Just like in a single master glyph, each outlines has a certain number of nodes — but in a MM glyph, each node does not hold just one x-y-coordinate per node, but for every node there is one x-y-coordinate per master.

ok, i see what you mean...
thanks for the reply

This is what happens when i blend them to build the MM...
The left ones are the originals, now look how diferent they get after that! Is this normal? Is this the right way to do things or am i doing something wrong? Could it be the point plancement on the rounded corners that causes this?

Seems there are two problems.

The first (and main one) is that the start nodes sit at different places. I’d use the position of the black weight for the light weight too. Obviously, FLS tried to reposition them, but since there seem to be other outline-incompatibilities, it inserted points and moved others around ...
[On Mac, Ctrl-Alt-T will highlight start nodes and co