Interesting though ....
As pointed out by Victor the TKC countermark was added to indicate the change in Trajan's status
ie his "elevation"
So obviously the coin was issued prior to Nerva's "deification"
Both portraits show the men laureate and the legend reads
ΑΥΤ ΝΕΡΒΑΣ ΑΥΤ ΤΡΑΙΑΝΟΣ ΝΕΡ
(granting to both men the same title of "autocrat")
The only real difference being Nerva gets top billing
So it has all the attributes of a joint reign of equals without any indication of Trajan holding the junior rank (Caesar)
which coins of both Titus and Domitian did previously
Methinks this one is a "keeper"