R1(config)# username USER1 password PASS1
R1(config)# interface serial0/0
R1(config-if)# encapsulation ppp
R1(config-if)# ip address IP-ADDR1 MASK1
R1(config-if)# ppp authentication pap
R1(config-if)# ppp pap sent-username USER2 password PASS2
R1(config)# username USER2 password PASS2
R2(config)# interface serial0/0
R2(config-if)# encapsulation ppp
R2(config-if)# ip address IP-ADDR2 MASK2
R2(config-if)# ppp authentication pap
R2(config-if)# ppp pap sent-username USER1 password PASS1