I split the blog using the multi-user feature of Pebble (the blogging software I use). The actual split was quite easy and took about an hour, the main part of the time spent to separate the posts into the blogs where they belonged.
After the split each blog reside in their own directory web-wise, as opposed to being in the root (/) of the server. This changes the URL you use to access the main blog (the one you are reading now). My main concern was therefore to keep existing links working. This was accomplished using mod_rewrite on Apache to make this blog reside at the root as well as in the new directory.
Having never really delt with mod_rewrite I had figure that out first but it turned out to be simpler than what I had expected. I just added a couple of RewriteRules to the virtual host and reloaded Apache. Simple enough (apart from the trial-and-error process of figuring out the quirks with the regular expressions and mod_rewrite flags).
I now use the following RewriteRules:
RewriteEngine on
RewriteLog /usr/local/apache2/logs/rewrite.log
RewriteLogLevel 0
RewriteRule ^/index.jsp /index.jsp [L]
RewriteRule ^/(.*)(themes/default|.css|.js) /$1$2 [PT]
RewriteRule ^/training$ /training/ [R,L]
RewriteRule ^/training/(.*) /training/$1 [L]
RewriteCond %{REQUEST_URI} !(^/lekkimworld/.*)
RewriteCond %{REQUEST_URI} !(^/(.*).css)
RewriteRule ^/(.*) /lekkimworld/$1 [PT]
Line 4 is used to make sure I access the overview page for the blogs. Line 8-10 is used to make sure that any request for a page not being explicitly for the /lekkimworld sub-blog is redirected to the lekkimworld blog. There is no need to handle the /training blog as well since RewriteRules are processed in a top-to-bottom approach so any requests for that blog has already been handled.
The mod_rewrite flags on the right [xxx] is used to signal how the RewriteRule should be processed. If no flag is specified the processing will continue down the chain. By adding a [L] flag to a rule, that rule will be the last one processed with the RegExp matches the request. [R] makes Apache send a HTTP code 302 back to the browser for a redirect. [PT] is used to pass the request through to the content handler without any further processing.