It can work that way. But ... two things to note here ...
USB3 is vastly improved over earlier USB ... but still not nearly the steady throughput of say eSATA II. Performance will not be quite up to what it would be with either internal/external SATA II connections. I do realize that especially laptops have dropped any eSATA ports on account of how supposedly USB3 can handle anything BUT ... that's not quite the truth with video files.
And ... using one SSD for OS, programs, cache, previews and such and ONE USB3 external for all footage/render usage will have some bottle-necking going on. If it's a laptop, there may not be a way around it ... but if desktop, anything such as even a third USB3 drive with direct to mobo connection would help. Splitting up tasks that involve disc access read/write operations helps immensely, which is why the tweaker's page so highly recommends four+ hard-wired discs or SSD's ... or preferably, both RAID SSD system drive and RAID array for other purposes. One disc for OS/programs, another for previews/cache, another for footage, project files on one or either the pv/cache disk ... that's the kinds of arrangements that can help.
Neil