1. the same network two ip not tied to a machine.
Because this segment will generate two routes, two routing packets corresponding to the same network segment, and selects only one route actual ip routing (estimated to be in front of that choice a) away from a card. So that regardless of which network card to the internal LAN ip packets from one network card is always out in reply. In this way it is possible to see the other side of the issue ipa message of receive a response ipb.
2. ip two different networks can be tied on a machine. But if there are other paths of these two segments interlinked (such as the middle of a gateway zz), may be abnormal service.
Such as configuration xipa and yipb. xipc access xipa and yipd access yipb are OK to. But if xipc through another path, such as gateway access yipb zz would not make sense, because the return message is returned to xipa.
Why does not the default choice of routing packets to the network card. Because the routing work at three, when ip routing protocol can only be selected with the purpose of ip three, even four ports can not see, but can not see the card source source source ip packet. Unless a similar traffic policy iptable this stuff.
The demand itself is nothing unreasonable, but the agreement itself is so layered.