1. If you are restricted for movement inside the trap bar, then handles are an option.
2. Sometimes, balance can be an issue, what I mean is- if you place your hands too far forward or too far rearward it can make it a challenge to keep the bar level, particularly when you are fatigued.
To fix this I've noted the best position for me and then wrapped some PVC tape (a couple of MM's thick) around both handles as a guide.
Other than that, single handles are good for doing suitcase carries.
All the best, farmer's are a fantastic exercise and a really superb finisher.