I have this issue and its a bloody annoyance but finally sorted it today!
Much like you I had 2 dreads that had formed into a matted clump at the roots.
Thankfully, I was able to find the area where they had joined, about an inch long section from the roots.
What I did was find weaker spots and wiggle my little finger in between toseparatethe dreads a little.
Doing this several times up the knot till I was able to pull away small amounts of hair from the bottom of the hair, slowly working my way up the clump.
About 45 mins, some popping/snapping sounds and a headache later the clump is no more and I have twoseparateddreads :D
Only bad thing is they both lost some form in the splitting and I lost a few bumps I was rather fond of but Im sure the now loose hair will re-dread back in quickly.
Unfortunatelyfor me one of the two dreads has combined with another set of hair which is well knotting but unlike the above case there is nodefinitivejoin between them that I can find. (I think that attempting toseparatethem would result in destroying both of them)
Quite tempted to allow them to join and make a fatty as I think separating them would be a far worse headache