You can make two shooting heads or WF lines from a single DT line, floating or sinking.
I cut them in half and use a loop to loop connection to 50 ft of
Cortland 444SL running line. Buy the running line in the diameter you want for your 3 wt. line.
Since the spools of running line are 100 ft., you get two sets of running line from each spool which matches the two sections of DT line. Since the running line can be used used with other wts. of line, once you make the loop connections, you can use it for other shooting heads.
Because you have attached the DT to running line, you can shoot the line just like a wt. forward line and you can carry much more backing on the reel than with a complete DT line. This set up gives you the mending and delicacy of a DT plus the distance casting of a WF. This is what we used to do with all DT lines before WT forward lines were developed.
There are three ways to make a loop at the end of a fly line. I use the first method.
Method 1 - Remove the plastic coating from the end of the line by soaking it for a short time in acetone. Use you fingernail to strip off the plastic. Take the dacron core and loop it back on itself and the still plastic coated section of fly line. Now use a fly tying bobbin holding unwaxed 3-0 nylon thread to whip finish the the loop you have just formed. The whip finish should extend partially over the dacron-dacron section and dacron-plastic section. Then coat the whip finish with pliobond.
You whip finish by holding the formed loop in your left hand between thumb and index finger and the overlapping dacron and fly line section in your right hand the same way. Now hold the nylon fly tying thread between your fingers as well and let the bobbin hang down. Start the bobbin swinging so that it swings around the two pieces of dacron core between your fingers and then continue to *wrap* the dacron with the bobbin which is now spinning around the dacron core. Note that the bobbin must be tight to prevent the centrifugal force from extending the thread. I usually wind the thread several times around one of the bobbin posts before bringing it through the bobbin tube and this prevents the bobbin from feeding thread as you whip finish.
When you have wrapped enough thread to secure the junction, then use some tippet material to form a loop, lay it over the wrapped section, wrap over the tippet loop about 6 times, then cut the thread, put it through the tippet loop, and use the loop to pull the thread underneath the 6 wraps you have just made. Cut off the tag end of the tying thread. You have just whip finished a loop connection. Coat the whip finish. I prefer Pliobond because it is more flexible but you can use Aquaseal if you don't want to buy Pliobond.
Use the two loops connections on each end of the tapered and level line to connect to each other. I have used this method with 2 weight running line and half of a double tapered line to make my own weight forward line without any lost or damaged guides. Make the loops as small as you can so that minimal energy is lost during the cast.
Method 2 - Remove the coating from only the running line. With a needle form a tract through the end of the DT line and out the plastic core. Use a looped piece of tippet to pull the dacron core of the running line through the tract of the DT line. Tie a nail knot using the dacron core of the running line around the DT line. Coat with pliobond.
Method 1 allows you to change out fly line with a loop to loop connection. Method 2 is permanent and is a smoother connection.
Another method of loop formation is shown by Global Flyfisher
loop to loop connection.