There are some pagers that will receive up to two seperate frequencies, and even have a priority setting that will check Channel 1 every few seconds if you put it on Channel 2, but it doesn't "scan" in the same manner that you'd expect a scanner to.
There are a handful of scanners that have the fire tone-out feature (for two-tone sequential paging), but a majority of them don't.
I don't know of any scanner that I've seen without a way to input freqencies, almost always through a numeric keypad. Scanners ARE programmable (albiet, some easier than others), not just pre-programmed. Perhaps you're thinking of something other than a traditional scanner?