Is is practical to have one interpreter to translate both directions?
Generally yes, assuming the interpreter is fluent both ways.
When not face-to-face, is it reasonable to have an interpreter on standby for telephone (conferene) calls?
It's reasonable. Just make sure that the phone setup will work. You will have to come to agreement on whether and how the interpreter should be compensated for stand-by time (i.e. periods during which the interpreter is required to be available but is not actually working).
Could one interpreter cover an extended day, assuming the work is intermittent. Or would it take a minimum of two interpreters?
Assuming you're talking about consecutive interpreting, one interpreter should be able to cover an extended day of intermittent work. Again, you will have to come to an agreement on compensation (overtime?).