Merging two internet bandwidths and load sharing

I have a large network with over 1600 pcs divided into 5 IP subnets, which are connected to a core switch. The subnets are 172.21.0.0/16, 172.22.0.0/16, 172.23.0.0/16, 172.24.0.0/16 and 172.25.0.0/16 and the core switch is the one doing all the routing between this subnets and its ip is 172.20.0.1/16.

Now I have two internet bandwidths coming in. One is over RF and is 1Mbps RAW and another one is 512 Kbps through VSAT. Both of them have different IP Pools. The Bandwidth-One has ip pool of 202.160.xxx.xxx/27 and the bandwidth-Two has ip pool of 202.141.xxx.xxx.

Now I want to merge this bandwidth in such a way that the total BW avlb. to the users is 1.5Mbps and whenever one of them is down the load is switched to other one automatically.

I have 2 PCs with redhat linux 9 and squid installed on it. Both of them have 2 NICs one connected to internal VLAN and other having live IP.