Is your Slackware machine going to be acting as a router for other machines behind it, or are you just looking to utilize both internet connections for the machine itself? In either case you may want to look at various iptables rules related to 'bonding' or 'load-balancing' internet connections. There are several posts, if I recall correctly, in the Slackware forum about this very topic, but you'll likely have to use the
Search Feature to find them.