Can we connect 2 5G modem with same ip address for internet in FortiGate Firewall 61F

You are looking for SD-WAN.

Here you start:
https://docs.fortinet.com/document/fortigate/7.6.7/administration-guide/889544

Hi ​@ameersultan 

It is not recommend to connect two modems with the same IP range (192.168.0.1/24) to the same FortiGate . This creates a routing conflict as Both modems would be on the same subnet (192.168.0.0/24)
Recommended Solution: Use SD-WAN with Different Subnets

Step 1: Change the Modem IP Ranges
You must change one modem's IP range to a different subnet. For example:

Modem 1: 192.168.0.1/24
Modem 2: 192.168.1.1/24 (or any other non-overlapping subnet)

Step 2: Configure the FortiGate Interfaces
Step 3: Configure SD-WAN for Load Balancing
Step 4: Configure Health Checks
Step 5: Configure Firewall Policies