I think I solved it. I had the exchange accounts setup with the 
serveradress pointing to www.mydomain.com/webmail/rpc.php
I then stumbledover the "Alias /Microsoft-Server-ActiveSync 
/var/www/horde/rpc.php" in the Wiki and implemented it accordingly. I 
then changed my setup so that the serveradress just points at 
www.mydomain.com and what can I say, things seem to work like a charm 
for now. I can sync contacts and calendars for 2 users without problems.

I dont know why not having the alias setup wreaks havoc on the sync 
but it does when you try do have 2 Exchange accounts, with one it 
works fine.

I turned Provisioning to "Disable". Reset the Activesync devices and 
resynced. It still synced the wrong adress book. But the Iphone forced 
me to use a new PIN Code after that ("Force device to usa a PIN" was 
set to "no"while provisioning was turned on). From there on the Phone 
wouldn´t let me use a simple code. Disabling the code was disallowed. 
Even deleting the offending Exchange account didn´t help. I had to 
turn provisioning back on ("Force device to usa a PIN" setting to 
"no"). After that the simple code option was back...

It looks like something is going wrong with the device provisioning 
between the two users.  Can you see if setting the:

* $conf[activesync][securitypolicies][provisioning]

value in the general horde configuration to "disable" fixes this for you?

The problem also exists for calendar data. So if user 1 adress book 
gets synced its user1 calendar,too. When user 2 adresses get synced 
its also user 2s calendars...

I turned of the caching in Horde resetted the Activesync devices in 
Horde rebooted the server and resynced the phone and the adress book 
of user 1 is back, but again for both users. At least it does not seem 
to be problem with the Horde caching.

Something truly weird is happening. Without reconfiguring or resetting 
activesync devices in horde. I switched of all syncs on ios for user 2 
and turned sync of calendar and contacts on and off for user 1. Now 
user 1 gets the adress book of user 2 and not his own?! Resetting the 
activesync devices in horde and resyncing still gets my user 2 adress 
book... Does Activesync cache its sync data so that somehow the wrong 
cache is accessed?

I have selected the personal adress book of user 1 for user 1 and the 
personal adress book of user 2 for user 2 for external sync. Both 
users do a proper sync request and get the personal adress book of 
user 1 and nothing else. I sent you the log via email. I gave up 
anonymizing it (and I don´t want to post it with personal data online) 
as it is multiple thousand lines long and I don´t know which parts are 
of interest for you.

Are you *sure* you have unselected the first user in the display 
options? I've tried this on 3 different iOS devices and 1 android 
device and it works as expected.

In the AS log, do you see the second user's SYNC requests? If you post 
the log I can take a look.

There are no errors in the Activesync log. I have selected for user 2 
the  personal adress book for external sync. User 2 logs in without 
problems and Horde hands out User 1 adresses book?! No problems logged 
in the Horde log.

I have iOS 4.3.4 and have just tried again and it still happens. User 
two gets User one data... I will now turn on Activesync logging and 
post any errors here....

I can't reproduce this on iOS 4.3

I added a second "Microsoft Exchange" account on the device, for a 
different user. After the sync was complete, selecting only the Group 
for the second account will show me only that account's information. 
Switching the Group to show only the first account, goes back to 
showing me only the first user's information.

Mentioned in Ticket [#10363]:

I use an Ipone and sync to Horde with Activesync. If I have 2 Exchange 
Accounts on one phone which sync to the same horde server but with 
different users, the second user will get the sync data (contacts and 
calendar) of the first one. I don´t know whether that is a problem 
with horde or with iOS