I have tested this on CentOS 7 with both axel 2.17.11 and an old build of 2.17.6 (to see if it might be a regression)

Also tested on macOS 12.6.3 with axel 2.17.11.

The interfaces option of axelrc does absolutely nothing.

The CentOS machine has a single interface with 5 public IPs on it.
interfaces = xx.xx.xx.90 xx.xx.xx.91 xx.xx.xx.92 xx.xx.xx.93 xx.xx.xx.94

The Mac has two interfaces, each one is a different ISP. I tried it both ways:
interfaces = en0 en1
interfaces = 192.168.0.20 192.168.1.251

I tried separating with commas instead of spaces. Then I tried tabs instead of spaces. I even tried invalid stuff like interfaces = poop, there is no error shown.

Regardless of what I put for the option, only the default route is ever used to make connections (verified several different ways).

This is output of curl on the Mac testing both interfaces. It is not my system, I think has to be a bug in axel.

MacPro:Desktop mike$ curl --interface 192.168.1.251 http://checkip.dyndns.org
Current IP Address: 76.250.xx.xx

MacPro:Desktop mike$ curl --interface 192.168.0.20 http://checkip.dyndns.org
Current IP Address: 75.212.xx.xx