How to join 2 overlapping planes I create 2 rectangular planes. Then I align it together. Now I want to join it to become one mesh. How?
It is likely that there are easier ways to achieve the effect you want. However, here's how you can combine the two planes:

1 - Select both of the planes and join them (ControlJ). The two planes are now one object, but we need to take care of the overlapping ends.
2 - Enter edit mode (Tab)
3 - Select the vertex editing option:

4 - Move the two inner points towards the other planes:

5 - Select the two vertices and merge them (AltM + Enter)
6 - Merge the outer corner vertices the same way.

You should now have this:



Note that merging the planes in this way is a bit tedious. Consider using another technique to model your object that does not require joining.