BML commands can be compounded together in blocks. For example, to have your character both raise his eyebrows while nodding, a BML block could look like this:
<face type="FACS" au="1" side="both" amount="1"/><head type="NOD"/>
There is no limit to the number of BML commands that can be compounded together. Either BML command could be explicitly started or timed by adding the appropriate synchronization points, such as:
<face type="FACS" au="1" side="both" amount="1" start="2"/><head type="NOD" start="4"/>
which instructs the character to move his eyebrows at two seconds, and nod his head at four seconds. The synchronization points can also be used by adding an id to a BML behavior, then using that id to synchronize other behaviors. For example:
<face id="foo" type="FACS" au="1" side="both" amount="1" start="2"/><head type="NOD" start="foo:start+2"/>
Thus the eyebrow raise has an id of foo, and the head nod will occur two seconds after the start of the foo behavior.The id is unique to each behavior block, and thus the same name can be reused on a different behavior block.