Okay, I have to explain the whole: HAVING TWO DIGIVOLUTION LINES, too you:
Your crests are the main source of your Digimon digivolving. After a time they may even become apart of you so that you may not need the crest in order to let your digimon digivolve, it depends on how close you are to your personality trait. Unlike in Digimon Adventure, there is a fine line between good and evil powered through your crest.
If you go against your crests personality trait, then your digimon is more than likely to digivolve into its second evolution line. For example if if I were to choose the crest of friendship and I was constantly falling out with my comrades then my Gabumon would evolve into its second evolution line instead of following the original Garurumon line. It would digivolve to say a Dobermon.
In otherwords, the second line is the darker line to follow. You see yourself growing more and more distant from your crest to the point where your digimon is digivolving puerly because of will, power and want of its tamer, not through your chosen personality trait.
So, your best bet is to make your second digivolution line a darker side of your digimon, but you don't have to if you don't want to. If you do not give your digmon a second digivolution line, I will asume that you plan on following the good and just.
Your digimon may even digivolve into its second evolution line for the first time if your character plans on developing through out the RPG and isn't in touch with his/her chosen trait yet.
Okay then. Thanks for clearing that up. I am going to introduce one
EDIT: Done. I introduced Dark Tyrannomon as my second line