Hi
On seeing the attachment i feel its unlikely that your second isp will route the ip block provided the by ISP - 1 in their network.
If you have your own address space available with you it can help a lot and solve most of the operational worries here in this setup.
If thats the case you can have both your F/W outside and the Ethernet interface ips on that particular subnet which can be easily routed by both the SPs.
As far as the load shedding is concerned you can make use of backup interface and backup load command which will start shedding the traffic to the interface mentiond in the backup interface command.
Currently are u running any kinda multilink over there with your ISP -1 ?
if thats the case you can point the ISP-2 connected interface as the backup interaface and also the load values as per your desire.
Also would suggest to check this link which discuss about having BGP in multihomed network..
http://cisco.com/en/US/tech/tk365/technologies_configuration_example09186a00800945bf.shtml
regds
