I think next year Spurs will get a pretty high draft pick. That will be the time to get the second star. A lot of people are projecting the Spurs to win a lot of games next year, but historically this hasn't been the case with "generational" talent in their rookie year.
Mavs during Luka's rookie year were 33-49
Cavs during Lebron's rookie year were 35-47
Sonics during Durant's rookie year were 20-62
Pelicans (Hornets) during AD's rookie year were 27-55
Spurs were amazing during Duncan's rookie year, but Duncan was 2 years older than Wemby is now and had a decent team around him including 32 year old DRob. Orlando during Shaq's rookie year went 41-41, even Chicago during Jordan's rookie year went 38-44.
Also of note, the likelihood of those 4 players staying with their franchises I think was directly influenced by the player(s) they drafted in the following year, while the team was still bad. The Cavs drafted Luke Jackson with the 10th pick after Lebron, the Mavs lost their pick in the Luka trade, and the Hornets drafted Austin Rivers 10th after AD's rookie year. Lebron, AD, and (possibly) Luka ended up unhappy as they dragged their teams into the postseason year after year but could never get the requisite talent to get over the hump.
OKC drafted Westbrook and Harden after Durant, ended making a Finals run, and then even though Durant left for the Warriors, very few people would say that OKC didn't have enough from a talent standpoint.
I think Wemby has a higher ceiling than someone like Luka, but I don't think he's as advanced as Luka or certainly Lebron coming into the league. We will most likely get 30-35 wins next year, which has us picking 5-8 next year and that will probably be the most interesting Spurs draft in the last 20 years.