“Legal” in this case would mean AHJ approved. Therefore, it would depend if the AHJ allows it.
As far as I know, there is nothing in NFPA 13 or NFPA 72 that explicitly prohibits placing the two within close proximity to each other. In fact, if one activates, it already does the job of protecting the building/occupants of a single fire.
I would be more concerned about how the smoke detector would obstruct the spray pattern of a sprinkler head (see Chapter 10 of NFPA 13 for standard sprinkler heads). Certain provisions require sprinkler heads to have clearances so that the spray pattern is unaffected – if a fire were to start, a spray pattern that is not according to its intended design will make a difference in how it can suppress the fire.