[CentOS] 2 routes & 1 destinations

Hi,

At office, we have I ISPs.

I want to lightly monitor each link latency in order to decide several routing. For that, I have only one external server: 1 IP, it's an OVH dedicated server. The quick picture is http://s24.postimg.org/n3436z64l/defaul_route.png

Default route is via ISP1. If OVH-server pings IP1: - the request will go through ISP1: it's OK - the reply will go through ISP1: it's OK

If OVH-server pings IP2: - the request will go through ISP2: it's OK - the reply will go through ISP1: it's KO because the metric will involve ISP1

How could I tell the Office gateway (CentOS 6) "IF you reply to a ping request from the OVH server that came on IP2 THEN route it via IP2"