You can use both links simultaneously, but only by statically routing some traffic over one connection, leaving all other traffic to use the default route through the other router.
For example, assuming your DSL router is at 192.168.1.1 and your Cable router is at 192.168.1.2, you can set your Mac's default route to 192.168.1.1 (the DSL line) and tell the system to manually route specific subnet's over the cable modem via the route command:
route add -net 18.104.22.168 netmask 255.255.0.0 192.168.1.2
This command will route everything for 22.214.171.124/255.255.255.0 through the cable router.
Of course, the trick is knowing which traffic is best suited by each connection. There are routers that can handle that via BGP, but I don't know of any home router that can handle that.