Quoting an user:

"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:



That's it.