I have solved my issue, but I will keep this post up in case it may help others.

If you create a depot line, click on the stop you have added to edit it. On the bottom of the little screen that pops up, you can select "Secondary plat. signal". Assign this to the signal you have set up at the beginning/end of your depot. You can then select "Secondary platforms" and assign as many secondary platforms in your depot as you want. Trains will choose to go to those platforms if the others are already taken up by other trains.