I have 2 paths for the internet, I have downlink on a device and the
uplink using a different device.

how do I set the router/gateway (or what ever I need) to accept the
incoming traffic from one device, and the outgoing traffic to use the
other device, and which one of them will be my default gateway for the
router, do I need to use 2 routers, and if yes, which one of them will
be the gateway for the clients?

Thanks in advance,

Ali