Finally figured it out on my own; multiblogs could bear with a bit more commenting. ;)

Here's the code worth paying attention to, in case anybody else gets stuck like I did:

For the first blog on in multiblogs.php, which is by default laid out normally, the settings are declared in the first part of the file. At the end of these settings are some commented-out lines that say:

# Additionnaly, you can set other values (see URL params in the manual)...

# $order = 'ASC'; // This for example would display the blog in chronological order...

(Unfortunately, the manual, which would have probably saved me much experimentation, is down for me, but that's another story.)

Immediately after, I added the $posts setting, as follows:

# Additionnaly, you can set other values (see URL params in the manual)...

# $order = 'ASC'; // This for example would display the blog in chronological order...

$posts = *NUMBER OF POSTS HERE*;

For the other two (or more) blogs - the "auxiliary blogs," if you will - there's a bit of code for each of them that goes sort of as follows:

$BlogBList = & new ItemList( $blog,	 $show_statuses, '', $m, $w, '', array(), $author, $order, $orderby, $posts, '', '', '', '', '', '', '', $unit, $timestamp_min, $timestamp_max );

As you can see, there's a $posts in there, too... You can simply add a =*NUMBER OF POSTS*, with NO semicolon since its one in a series of settings. For instance, for one of my auxiliary blogs for which I wanted to display 10 posts, I changed the above code to this:

$BlogBList = & new ItemList( $blog,	 $show_statuses, '', $m, $w, '', array(), $author, $order, $orderby, $posts=10, '', '', '', '', '', '', '', $unit, $timestamp_min, $timestamp_max );

Hopefully when the docs are working again the rest of the many settings will be easy to figure out.

Two years later I just find this article. Thank you very much. I couldn´t figure out how to show up in the multiblogs.php page one article per blog. And the question was already answered.