Set up a small capsule condenser microphone about 3 feet for a solo or overdubbed banjo. If your room acoustics are not the best, use a cardioid (heart-shaped) pick-up pattern mic, otherwise an omnidirectional pattern is preferred for capturing the banjo in its environment.
Place a large diaphragm condenser about 6 feet from the banjo, with the same consideration for polar pattern based on the sound of your room. A large diaphragm microphone will exhibit darker tone color on sounds arriving off-axis, that is, room sounds and reflection. Do not combine the mic signals. The respective distances from the banjo will cause phase cancellation issues.
Use a figure-eight pattern condenser microphone on an instrument like an acoustic guitar if you are recording banjo in an ensemble. Place the mic to balance the sound of both instruments, likely closer to the guitar. Isolation will be difficult no matter how you set up mics, therefore use the figure-eight pattern to advantage to capture the full banjo sound.