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
I crosspost with comments on both sites, and for autobiographic content I'll keep at it for a while because a few of my RL friends are on LJ and won't move. I also still have an active flist on LJ, and many communities I belong to who don't exist here, so. My free account over there is still getting some usage.
and thank you for your message.
It's easier for me to keep everything in one place, but I worry that it's less convenient for people who aren't inclined to visit that place, so: poll.
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.
I do think that continuing to have comments split, and having LJ as the locus for neither set, has caused commenting to decrease somewhat. OTOH, the few readers who have never liked or understood LJ are happier now that there's a blog-shaped place to comment. *throws up hands*
It bugs me when people don't at the very least include a link to the other version of the post -- but, as you point out, that is awkward to do in reverse (you have to manually add the LJ link to the DWth post), so it's not a great solution.
I'm much less likely to comment on a post if I've seen that someone already brought up my point (or something close enough to my point), even if I might have expressed it differently and any resulting discussion if I had commented might have spun off in a different direction. So I could imagine that under some circumstances, consolidated comments might actually reduce the overall number of participants and diversity of perspectives. It's hard to assess, but I wouldn't be surprised if I was actually getting more participation in comments for at least certain kinds of posts, now that I'm cross-posting than before when I only had an LJ. But of course there are the tradeoffs that you mention.
If everyone's reading the same post, then if you had spoken up, any interested parties would have heard you, just as the first commenter was heard. If post is split up, there's no guarantee that you'd be on a different site than the first commenter, and there's no guarantee that anyone who spoke up would be heard by any of the potentially interested parties reading the post.