If the 2nd milestone is funded before you start with the second half, it's perfectly fine. But you should insist on that milestone being funded before you start on part 2, not afterwards.
And make sure you define "half" as what it really is. E.g. in writing, clients sometimes propose 50% for the first draft, and then 50% for revisions/edits. However, the first draft is really 80% of the entire job, so there is a chance of them running off with your draft at pretty much half the price.