Hi,
Will, you need to put the both ip addresses for both ISP's in the crypto map of the remote site, this can be done by:
crypto map set peer
Also on the site where you have the two ISPs you need to apply the same crypto map on both interfaces facing both ISPs.
P.S: You will need to configure SLA on the ASA facing the two ISPs (which I asume you alredy have for ISP failover).
HTH,
Mo.
