I know not many here use FontForge. My experience with Glyphs is very limited, because I haven't experienced it much, in the moments when my daughter "gives" me her MacBook Pro. Consequently (at least until I buy an iMac) I continue to use FontForge on my Linux machine.
I created a Marl-to-Mark lookup, added a subtable and then a first anchor class Anchor-a.
I placed a Base anchor above uni0308 (Combining Diaeresis) and a Mark below uni0304 (Combining Macron) and placed them in class Anchor-A.
In the Anchor control window everything appears to be regular, I see the two glyphs appropriately located in the two categories (the first in Bases, the second in Marks) and I can change the alignment height.
However, when I give the Build Accented/Composite Glyph command the glyph is produced according to the FF default and does not follow the specific lookup positioning.
Is there something wrong or omitted in the procedure I follow?