A screenshot would greatly help here because really...it depends!
If your lettering is meant to be read easily (a heading for an article), I would not mess with the words too much as to keep legibility. It's ok if the length of the words is different. If you had more than 2 lines, you would have to check if your rag is balanced (you don't want to have a staircase effect on the side where your words are not aligned but it doesn't have to look aligned either.) Something like below:
and not something like this :
If your lettering is meant in for an expressive display (a logo let's say), occupying all the space might help the legibility in small sizes or just make a bolder statement by the overall shape the words are creating, maybe even adding to the idea you want to communicate. In that case you can try playing with font size, font weights (condensed, extended, light, bold, etc.) and tracking. Stretching the type is not usually a good idea as it messes up the letterforms. If one of the words is much bigger than the other, consider if the emphasis seems logical.
If you want to fill the space without messing with the letters, you could also possibly add a graphic element in the leftover space: an arrow, dots, etc. Always related to the subject you are illustrating.