If your router supports 'metric' routing that it can be done. Its a common feature in most cisco routers. However I am not much experienced with SOHO routers.
For instance a routing entry is like this (on a PC)
'RemoteNet' mask 'RemoteMask' 'Gateway' 'Interface' 'Metric'
203.200.199.0 mask 255.255.255.0 192.168.0.1 1
203.200.199.0 = RemoteNet
mask = mask (required keyword)
255.255.255.0 = Remote Subnet
192.168.0.1 = the ip of the device on ur network which is your gateway for this network, in this case IP of the interface of your router connected to ISP1
1 = Metric (used to either denote hop count or simply to denote how close the remote network is to your router. THIS is the parameter that is to be used to priorotise routing also to swicth to other route if this one fails
for ex:
0.0.0.0 mask 0.0.0.0 192.168.0.1 1
0.0.0.0 mask 0.0.0.0 192.168.0.2 100
The above 2 routing entries are both routing to 0.0.0.0 (everything else or simply everything/internet for all practical purposes)
The first route uses the device/ip 192.168.0.1 with a metric 1. So this would be the route (isp) that is used for all outbound/inbound traffic.
The second route also points to the same remote network which is 0.0.0.0 so between these 2 routes the one with metric 1 (lower number) takes precedence and routes all traffic as long as 192.168.0.1 (gateway 1) is reachable. The moment it is down the second route kicks in and starts routing traffic to the second interface/ISP.
As you can see, this assumes that your router has atleast 2 wan interfaces which can 'physically' connect both ISPs even though only one is used at a time.
Hope this helps.