Setup for 2-pfsense loadbalance/failover?

[feait]

We have an installation with 2 3-port PFsense hardware boxes, connected with both PPPoE links to the same ISP - the setup looks like this.

WAN1                                WAN2
      |                                      |
–---------      sync (Opt1)  ------------
|pfsense1 |-----------------| pfsense2 |
-----------                        ------------
      |                                      |
      LAN1                                LAN2
      |                                      |
  ---+---------------------------+------

Both pfSense routers are using PPPoE to the ADSL wan via bridge-mode ADSL modems.  I can figure out how to do failover from left to right using CARP (tested in VMware) but
is there any tutorial/guide on how to setup CARP failover and inwards & outwards round-robin load balancing?  Can it be done with this type of setup or would we need pfSense hardware boxes each with 2 WAN links, a LAN link and a Sync(Opt1) link for CARP?  As pfSense only supports 1 PPPoE link per instance, this would mean some changes to the modems as well if each pfSense box had to support 2 WAN links.

Any help or pointers to guides for this would be appreciated.

Regards,
Greg Hastings
Systems Administrator
FEA Ltd