Lets say.. 50 lb power pro and 60 lb ande mono? or something similar to this?
This is precisely what I have researched and worked on for the past two years. Initially, Sergio and I were trying to make hollow core/needle insertion work with the 60# braid and 60 # mono combination. Needle insertion into the lightest hollow core braid (60#) is akin to brain surgery. No one would want to do this while surf fishing. Additionally, hollow 60# braid is so incredibly limp that when in a wave wash a slack line would tie itself into knots !
The solution is to weave regular solid braid over the top of mono which in essence creates the same thing as a hollow core splice. The best knot I've found for this is the Sebile knot. After about a dozen practices you will be able to tie this knot in fishing situations, After about 25 practices you will be able to tie it quickly under all conditions (even in the dark). By far the best way to learn this knot is with the Youtube video from Salty Shores. .....with one important modification. When using lighter line lines (like 50# braid/60# mono) the first crossing wrap of the braid will scissors right through 60# mono. I rectify this by starting the knot with four simple overhand wraps of the braid over this mono. This provides a cushion effect and eliminates this problem. Also, at the finish of the knot, it's best to limit the overhand knots to two turns. Three turns will many times prematurely cinch before they are pulled down. It's important to live a slightly long tag end of both the braid and mono (@3/8 inch) because the knot will elongate slightly under the first several high stresses. A drop of super glue on the finishing overhand knots only
will help ensure that the finishing overhand knots don't untie (they are under no tension).
This line connection beats any swivel connections I've tried. It's enabled me to go from 50# braid to 30# braid while fishing for big roosterfish, crevalle, etc.. That line diameter reduction has given me the extra casting yards that are very important in tropical Pacific surf fishing. You also never have to worry about cracking into your tip guide with a metal swivel. It's very durable and I've sometimes I've used a single knot (no retying) for over a week of hardcore Pacific surf fishing. I've introduced this line connection to lots of guests at my fishing grounds and many of them have carried it over to their home waters. - HPD
BTW - you can do a very effective braid line splice by tying this knot onto both ends of a short length (6-12 inches) of mono.