If the DSL operates using the dialler (in the UK, ADSL with a USB modem is generally run that way, with the dialler options set to the required parameters), then you could set that not to be automatic, and then the default would be "always on" cable, with the option to "dial" the DSL, which will probably override the other connection.
I'm comparing with my backup modem dialups, which always override my cable connection - handy if I need to send an email on the old connection.
If both were active, lets say two routers, then it may be possible to balance them, but the details available are very sketchy.
http://www.geekswhoknows.com/articles/load-balance-two-internet-connections.htm - this seems to cover using two ethernet adapters, rather than two connections, though it might also work with a USB and an ethernet.
With two exit paths on the same LAN, the only thing that seems to apply is the "ROUTE" command.
To be honest, if your backup connection has the option to be made "dial manually" (from connections in control panel), rather than dial at startup or dial on demand, that that is probably the simplest option.