Hi Sam,
I agree with Iñaki: application programs that call outbound proxies, don't have tight coupling to specific PI/PO Integration Servers. Application layer (program that calls an outbound proxy interface) is abstracted from integration layer (proxy runtime). Target Integration Server determination is done by proxy runtime based on combination of factors and application program is normally indifferent about which Integration Server will process corresponding request. This is the entire concept of application and integration layers segregation, so that application logic shall not take care or get involved in integration logic.
Refer to SAP Note 1334174, it contains details on how to configure outbound proxies to send messages to multiple PI/PO Integration Servers based on sender interface.
Regards,
Vadim