Modrewrite

I think You need to use external program for that.

RewriteEngine On
RewriteBase /
RewriteMap splitDigits prg:/path/to/splitter
RewriteCond %{QUERY_STRING} id=(\d+) [AND]
RewriteCond %{QUERY_STRING} name=(\d+)
RewriteRule ^attachment.php ${splitDigits:%1}/%2.thumb [L]

And the /path/to/splitter should be executable and should look like
#!/usr/bin/perl
$! = 1;
while (<>)
{
 print(join("/", split(//, $_)));
}

#!/usr/bin/perl
$! = 1;
while (<>)
{
 print(join("/", split(//, $_)) . "\n"); #forgot the newline, sorry
}

I have a old php to send the thumbnails, but I want link direct to the thumbnails.

the .thumb is a .jpg

With this code I can put in the apache config and apache rewrite and convert:

http://thumbs.domain.com/attachment.php?id=12345&name=005.jpg

to this:

http://thumbs.domain.com/1/2/3/4/5/005.thumb 

??

I haven't tested that, but should work. Test it.

Do you know the code to do this:

#!/usr/bin/perl
$! = 1;
while (<>)
{
 print(join("/", split(//, $_)));
}


Without a perl, only with RewriteCond

No, thats why there's RewriteMap option.
RewriteCond/Rule only matches some patterns - do not modify them.