Andy Mangels said in this forum that Michael Martin is simply not a character writer. He is however a plot and politics writer.
A bit of a simplification/misquote...
I actually said (as I recall) that I wrote the majority of the character-based material and scenes, and Mike wrote more of the continuity-based and political material. Both of us had our strengths, and in rewrites, would strengthen scenes each other had written in ways that made them mesh well.
We definitely plotted the books together, sometimes with the bones of a plot from the editor.
A lot of the Romulan politics in Enterprise was pure Mike, but in other books, that might be different. I handled many of the Vulcan scenes, for instance.
I wrote almost all of the Bajoran stuff (as I recall) in DS9 Cathedral, and all of the Trill politics in Worlds of DS9: Trill - Unjoined, while Mike did more of the Gamma Quadrant segments and history of the Trill species in those books (respectively).
In The Sundered and Titan books, Mike wrote almost all of the Neyel material, while I did more of the Federation material. On the SCE books, because there was so little politics or continuity stuff, we balanced out very closely. So, it really all changed, depending on the project.
Hope that helps clear it up.