Adrian
I do not believe that it is supported for two IPSec VPN tunnels to originate on a device and both terminate on the same remote device.
If your objective is to have some redundancy (or failover) capability using both adsl circuits it may be possible to set it up if you do not peer from the adsl interface address. If you can establish the IPSec peering using the inside LAN address (or a loopback address) which could be reachable through both of the adsl interfaces then the single IPSec tunnel could use both adsl in load sharing or failover mode depending on how it was configured.
HTH
Rick
HTH
Rick