it should work but better to just get a load balancing router. less hassle that way since isang beses ka lang mag set up (sa router interface mismo). also, load balancing routers are not that expensive anymore (kung ang regular router say mga 2k, ang load balancer nasa mga 4-5k lang)
now that I looked at the image more closely, it wont work as well as you thought.
PC1 will only be using ISP1, if that fails it wont automatically connect to ISP2 since the gateway and its subnet is only for the router in ISP1. you might need to switch the manual configs of each PC if you want to switch to be using ISP2's connection
just get a load balancing router so it will do all the work for you