Dreamwidth now allows you to insert a link in the LJ post to its DW version, complete with number of comments posted on DW, but unfortunately it's not reciprocal. Alternately I could disable comments on one site and steer them to the other, to centralize discussion.
This isn't an issue with most of my posts, so I've put off doing anything about it, but occasionally I'll post something where a discussion emerges on one or both sites. I've found myself in replies referring people to a comment thread on the other site a few times recently.
Looking through my last few months of posts, and counting only those which have at least 10 comments on at least one site (though usually about half of them would be my replies), comments on Dreamwidth version edged out comments on LiveJournal by about a 5 to 4 margin (454 to 369). Though I'd guess that if I went through those same posts and tallied up number of commenters (anyone who's commented at least once), LiveJournal would come out ahead.
So should I do anything differently?
Should I centralize comments on DW and disable them on LJ?
Sure, works for me
15 (68.2%)
No, please don't - I prefer to comment on LJ
1 (4.5%)
But why? Two discussions are better than one!
2 (9.1%)
Text - such a primitive interface! I do my commenting via telepathy.
0 (0.0%)
I care more about convenient links to the crossposted entry -- can you get on that?
4 (18.2%)
If you're crossposting, what's your experience been with comments?
I centralize on one site, and have no regrets
9 (47.4%)
I centralize on one site, and I'm paying the price in less discussions
0 (0.0%)
I cross-link the DW post in the LJ entry, and it's been a positive experience
1 (5.3%)
I cross-link, but mainly for my own convenience - I'm not sure anyone else cares
4 (21.1%)
I don't cross-link or centralize. I do however recycle and watch my cholesterol.
5 (26.3%)
- Music:N.E.R.D., You Know What

Comments
as it happens, most of my primary fandom has moved to dreamwidth, because of synecdochic, but there are plenty of other friends who have stayed on LJ. and there are a few SG-1 notables who have said DW is a Do Not Want.
So i feel i should be both places.
at first there was a bit of opposition to having to post comments on the dw side of a crosspost, so i left comments enabled both places.
but i'm interested to see if there is an eventual migration of my part of fandom to DW, or not.
For the moment, i am content.