I have three Drupal 6 installations; one for a family site, this one, and one personal-interest site. When I initially set them up they were on separate hosting accounts. Now, because I've changed hosts and because none of the sites has massive traffic numbers, I've merged them onto one account. Since I've done that, my next step is to merge them into a single Drupal instance so that maintenance will be easier.
I haven't fully thought out all the ins and outs but the think the sequence needs to be:more...
- Update all installations to current version of core and modules