In alternate/parallel forms reliability you first have to create two parallel forms.
One way to accomplish this is to create a large set of questions that address the same construct and then randomly divide the questions into two sets.
Furthermore, this approach makes the assumption that the randomly divided halves are parallel or equivalent.
Even by chance this will sometimes not be the case.
The parallel forms approach is very similar to the split-half reliability. The major difference is that parallel forms are constructed so that the two forms can be used independent of each other and considered equivalent measures.
For instance, we might be concerned about a testing threat to internal validity. If we use Form A for the pretest and Form B for the posttest, we minimize that problem... it would even be better if we randomly assign individuals to receive Form A or B on the pretest and then switch them on the posttest.
With split-half reliability we have an instrument that we wish to use as a single measurement instrument and only develop randomly split halves for purposes of estimating reliability.