Whole idea is agregating all avaliable traffic to single logical link without changing source ip adress.
Best would be using some kind of Ethernet-trunk/-channel.
That would recommend of course an adequate switch at the ISP-side too.
the usual way is (OSPF-) ECMP, but this is working at the IP-layer.
if you use OSPF , you will have problem because, the metric is the bandwidth, and is one ISP have T1 and the other is 512 kbps, the traffic only pass to ISP T1.
you will need BGP.
or put static routes, and segment de users, but i very like more BGP but you will need contact the ISP for talk about the AS of BGP ,etc
in Wiki on this website have example for static routes