In the attic of my new house I have two PVC sewer vent pipes that come
up from the floorboards. One of them is run completely through the
roof. The other terminates about two feet up from the floor into the
attic. Would it be feasible for me to put an elbow onto this pipe,
extend it over and join it with the pipe that goes through the roof.
I would like to avoid putting another hole in my roof for obvious
reasons. I'm not sure about the codes in my area, but I imagine that
this would not be a problem. I would appreciate opinions.

If they're both really sewer-vent stacks, then last time I checked, you
could connect them. (subject to some rules involving flood-levels of
connected fixtures that aren't likely to affect you in an attic.)
Are you sure that's what they are, though?
It would really suck to find out after the fact that one of them's
actually venting your laundryroom, or a bathroom.