(On reflection, I think my subsequent comment actually answers the question pretty-much in toto so I'll put it here instead --dpb)
It ( unix ) still starts a new process only without the ampersand the calling process waits for the spawned process to complete before continuing...with the ampersand, the caller returns immediately after starting the process and continues, thus starting the second process irrespective of the status of the other.
The unix (or system or dos, they're all interchangeable) command in Matlab maps into a system API that has as a parameter the behavior of whether to wait or not; the ampersand is the Matlab syntax used in making that call internally as to whether to set the flag or not. The spawned process is still a separate thread either way; nothing is different there, only whether your code will or will not wait for the first to complete before going on to the next.
It is not possible to schedule the two processes unalterably in order if the ampersand is used with the interface as TMW has implemented it in Matlab.
You could achieve that effect by embedding the commands in a batch file and submitting that to the OS instead of the two commands individually; then they would run sequentially in the same process.