What about connecting a wireless router to the Cable then connect the
DSL to the Wireless router which are only $5 after rebate.
During initial setup, the router takes network settings from the
cable/dsl modem. Depending on where the OP lives and the type of DSL,
the DSL connection will be PPPoE or PPPoA and may require an account
name/password to connect. Cable uses a different structure and usually
does not require the name/password for initial setup. The OP could
certainly have two separate networks in his home or office using DSL
and cable; however, he cannot connect them together as he apparently is
asking. He could have two completely separate wireless networks
provided by two routers connected to each service also, as long as the
subnets are different. It seems a rather odd setup though, and it will
be interesting to know why the OP is paying two ISP's for the different
services.
Malke