Using synchronous communication with the two modems (modem-1 and modem-2), you simply cross-connect them by connecting modem-1's TX
to modem-2's RX and vice versa.
If your modem supports SYNC dial, you can just connect a local line to each modem and then dial the number of the line connected to one of the modems.
You can use the default HDLC encap on the router and configure addresses.