I looked into this some time ago.
There are tools available to help but to my knowledge nothing tested/demonstrated for Yunohost.
On my side, I want to make the mail server part redundant, and that’s already a little challenge to integrate with Yunohost.
Dovecot (the IMAP server) has a functionnality for real time synchronisation between two servers : http://wiki2.dovecot.org/Replication
For Yunohost, one would need also to replicate the LDAP data, and openldap also has some replication functionnalities.
With those two enabled, this should be possible to have a real-time replicated mail server, allowing to have two primary MX in the DNS, so that when a mail arrives to one, it is synced with the other (in both directions).
I think this can be implemented as a Yunohost application to be installed on both ends. I hope I have time some day to dedicate to this.
For the rest of Yunohost (the web app), this is more complex and I did not found yet a simple solution : in addition to MySQL, you need to replicate the file system and this is far less easy to do in real time. Happy to hear about possible solutions though.