How to connect two asterisk server as VOIP G.W and BAPX ?

You have to set a SIP or IAX2 trunk between your tow servers.

You will simply have go thru trunking concepts and use IAX or SIP whatever you want to setup that.
Regards, Aziz

we should have iax2 protocol

using switchvox product , u can set a voip provider using IAX2 and peer devices with each others also u can manage the dial plan for using extensions to make calls between2 devices

create a SIP trunk in both server for each ip and create outbound route to route calls in selected trunk.

They are two way : A-Either you create a trunk between the PBX and the gateway " but make sure not to mistake the parameters that are prepared in the PBX trunk".
B- Some gateways support SIP extensions , so they allow you to create extensions on it that you will use to send calls to FXO lines , so what they are doing is relating the SIP lines on the gateway that are registered on the PBX with FXO channels on the gateway

follow my steps first: create two trunk one in each server if we called the first server "main" and the second server "remote"trunk name="main" peer details host=dynamic type=friend contect=from-internal secret=enter any password canreinvite=no insecure=port,invite   Register String remote:password you choice@IP of main server Do the same steps in the main server>>> you should create out bound root in each server  root name =main-out Dial patterns prefix= for example "9" this number like if you are in main server and you want call anumber in remote server you dial9 berfore the number.
match number=XXXX represent number of number assignedtrunk sequence choose your trunk   if you want any thing about asterisk this is my email  

sip trunk or iax trunk betwwen
2- server