PPP PAP and CHAP example

in the configuration below, I have configured PAP and CHAP and it worked in both cases, however I used one router as Server and one as Client.

PPP Configuration
=================
PPP with PAP authentication:
in this configuration R1 is PAP Server and R2 is PAP client

R1(config)#int s1/0
 R1(config-if)ip address 1.1.1.1 255.255.255.0
 R1(config-if)#encapsulation ppp 
 R1(config-if)#ppp authentication pap
 R1(config-if)#exit
 R1(config)#username R2 password cisco

R2(config)#int s1/0
R2(config-if)ip address 1.1.1.2 255.255.255.0
R2(config-if)#encapsulation ppp 
R2(config-if)#ppp pap sent-username R2 password cisco

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PPP with CHAP authentication:
in this configuration R1 is CHAP Client and R2 is CHAP Server

R1#configure terminal
R1(config)#interface serial 1/0
R1(config-if)ip address 1.1.1.1 255.255.255.0
R1(config-if)#encapsulation ppp 
R1(config-if)#ppp chap hostname R1
R1(config-if)#ppp chap password cisco


R2#configure terminal
R2(config)#username R1 password cisco
R2(config)#interface serial 1/0
R2(config-if)ip address 1.1.1.2 255.255.255.0
R2(config-if)#encapsulation ppp
R2(config-if)#ppp authentication chap

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 I believe in real world there could be scenarios where there is one server authenticating multiple clients or multiple servers authenticating each others.

I guess in the scenario where one server is authenticating multiple clients , we might just need to add the following line on the server for as many clients as we have,  correct me if I am wrong:

RouterServer(config)#username client1 password cisco
RouterServer(config)#username client2 password cisco2
RouterServer(config)#username client3 password cisco3
etc....
but what about the scenario when 2 servers authenticate each other? can someone post an example ?

Thanks