Here’s a great video that shows you how to tie a double uni knot, which entails creating two unis and then sliding them together. According to the folks at Fly Fishing & Dreams, who made the video:
The double uni knot is an excellent knot to use when attaching lines of similar diameters. It is great for attaching backing to backing, monofilament to monofilament and even braided line to monofilament. It is easy to tie, very strong and has a very small profile, which enables it to pass smoothly through the guides of a rod.