"I cannot seem to figure out how to, for example, put down a whole note on one string of a quarter note on another *that start at the same time*. This makes it impossible to create a base line for a piece of music that isn't played exactly on beat with the treble part of the music."
Tablatures allows you to write your high and low melodies independently, thus you can achieve what you need in the following way.
Let's start by adding a note to the high melody:
Then, in the Notes Panel, we switch to Low melody, change the duration and enter the second note:
The most important addition in this release is a basic and experimental* support for GuitarPro 3-5 files (*.gp3, *.gp4, *.gp5). GP tablatures can be read, but they will be converted to PowerTab when saved, keeping only the first two scores since that is PTB's limit.
* this means that some files may not work. Feel free to report any problem you find.