The need to join two bolts together is common in construction and renovation projects. You can join any two externally threaded bolts with a standard or custom designed hex coupling nut. Use a standard coupling nut to join two bolts with the same body diameter and pitch. To engage bolts with different body diameters and thread pitches, bolts with left hand threads, or one bolt with right hand thread and the other left, use a custom coupling nut.
Measure the diameter and thread pitch of the two bolts you wish to join together with a bolt gauge. If the thread and diameter are the same, obtain a standard hex coupling nut with the same size diameter and thread pitch as the bolts. For example if both bolts are 1/4 inch diameter with 20 threads per inch use a 1/4 inch by 20 threads per inch coupling nut.
Grip the outside hex of your coupling nut with an open end wrench. Insert each bolt into the ends of the nut. Hand or wrench tighten both bolts as deep into the coupling nut as needed. If the depth of the bolts is critical, use a sight hole coupling nut. This coupling nut variation has a sight hole for observing the amount of bolt engagement.
Purchase a reducing coupling nut if the bolts you need to join have different diameters or thread pitches, such as joining a 3/8 inch with 16 threads per inch and a 1/2 inch with 13 threads per inch bolt. Repeat the process of engaging the bolts into the coupling nut as you did with same size bolts.
Some reducing coupling nuts are stock items at hardware dealers or fastener distributors. Others may need to be custom manufactured. Coupling nuts used for joining left hand threaded bolts or one left and one right hand threaded bolts are strictly custom manufacture. You can get custom quotations from most commercial fastener distributors and coupling nut manufacturers.
You can buy bolt gauges and coupling nuts at a hardware dealer or a fastener distributor. Coupling nuts are sometimes called "rod coupling nuts" because they are commonly used to connect threaded rods.