Default route in debian with two interfaces

To me, it looks like both DHCP clients spawned -- each for its corresponfing iface -- get the default gateway and race. Things happen this way that the one on eth1 reliably wins (for whatever reason).

The solution does not appear to obvious because it depends on your setup. Getting the default GW using DHCP logically means you do not care about where it is.

Based on this and this, I'd try adding

interface eth1 {
    supersede routers ""
}

to /etc/dhcp/dhclient.conf and see what happens. The idea is to supersede any routers supplied by the DHCP server reached via eth1 with nothing.