Closing, as this is a client limitation.

+1 for this option.  Another wrinkle of the single address book sync 
limitation is the Favourite Recipients address book.  Even if the user 
has only a single address book, if the ID of the actual address book 
lands after "Favourite Recipients" when sorted alphabetically, 
Favourite Recipients gets sent/sync'd and not the user's address book. 
  So, it's a crap shoot.

In my case I got this to work for a user by creating new address books 
until the generated ID sorted alphabetically before "Favourite 
Recipients", and then imported their contacts into that.

My provider (EuMX) is using Horde 5.2.1.

OSX does give errors. Here's what I get on my console

9/2/14 8:34:23.350 AM Contacts[37620]: [CardDAVPlugin-ERROR] 
-copyLocalETagsForURLs: relativeURI

/horde/rpc.php/addressbooks/<user>/contacts%3Afavourites/support%40eumx.net.vcf is not in 
/horde/rpc.php/addressbooks/<user>/contacts:favourites/
9/2/14 8:34:23.726 AM Contacts[37620]: [CardDAVPlugin-ERROR]
CardInfoFromVCardData(): Ignored vCard at 
/horde/rpc.php/addressbooks/bitbug/contacts%3Afavourites/support%40eumx.net.vcf because there is no UID 
property:
"BEGIN:VCARD VERSION:3.0 FN:support@eumx.net
EMAIL;TYPE=INTERNET:support@eumx.net
N:support@eumx.net;;;;
END:VCARD"
9/2/14 8:34:23.728 AM
Contacts[37620]: [CardDAVPlugin-ERROR] -_handleAddsOrModifies: could 
not parse vCard in 
/horde/rpc.php/addressbooks/<user>/contacts:favourites/

CalDAV bombs too. Every other time I start Calendar it loses all the 
events. It gets them back every other time, so Calendar alternately 
loses and resyncs calendar events from Horde.

Well, I have no clue how hacky the implementation would become, but I 
love the automatic multiplex as it is done for MS OL 2013 - the same 
for Mac OS would be great. just my two cents..

This is a limitation of the OSX Contacts app.

The only option would be for us to sniff out the user agent for OSX 
and multiplex the address books together like we do with 
ActiveSync/SyncML. Not sure if that's worth it to work around a bug in 
a single client though.

Mac OSX works fine with ical <-> calDAV, but contacts <-> carddav does not:

Setting up the account does work, but it syncs only the global adress 
book (In my case the GAL contains a singel contact) which is set up, 
not the users adressbook as intended. It doews not matter if  the 
pricipal URL or the abonnement URL is used during setup.

Mac OSX does not throw an error.