My hardware store only sells 8' cuts of 2x4s. How do I extend length horizontally with another 2x4 and support the joining best? Is there a joint or strong tie for this? I have three support legs spaced every 66". Should I just ensure all extensions occur on a support leg or can they be done elsewhere? This is for a simple workbench I am trying to build in my garage. I have no clue what I'm doing but a table seems like it should be an attainable goal to build. I can't find plans for anything longer than 8", which assumes one long board. I need one longer. How do I join them to extend length? What are strategies, hardware, etc.?
Why not simply construct a series of 8' long workbenches? This will give you more flexibility later on, if you need to deploy them differently (e.g. in an "L" shape instead of end-to-end.
8' benches are worth considering for a couple of reasons: the sheet goods you use on top will be that length. You'll get a slightly shorter span between legs, which will lead to more stiffness.
That said, if you need 132", there are a few options:
find a better lumber supplier. (Don't know where you are, but you should be able to find a lumberyard nearby.)
double the 2x4s. So the front will have 2-2x4s at 66"; put a 2x4 behind that butt joint (no plates/nothing else special) with 4' on each side. Put a leg under the 66" mark. (Don't worry about filling in the 18" gap on each end of the back 2x4, unless it upsets your sense of symmetry.) Screw the 2x4s solidly together.
for the top if you use sheet goods, you'll need 2 sheets. be sure to support the seam with a couple of 2x4s placed upright.
If you want it to be strong, the simplest way to extend a board is to sister it with other boards. For example, rather than using a 2x4, you could use layered 1x4's, with the breaks between boards in each layer offset from each other, laminated together with screws every foot and/or glue.
There are "glue joint" techniques for splicing boards end to end, but they're more complicated, waste some wood, and really require fancier equipment to do properly.
Why cant you put a support in where the two 2x4's meet, add a steel T to the support then screw each 2x4 to the T. This makes the support the main focus and provides needed strength