well think about it. Marvel universe and the supergirl flash and all that series know whats up so they link their shows with crossovers yearly so you feel like checking out the others.
In an ideal world there would cross progression between the 2 games but the key thing to remember is that Siege is now 6 years old. In order to have cross progression between 2 separate titles, BOTH titles need to have been designed with this in mind from the start. Implementing something like that after the fact can be impossible at worst, extremely difficult at best.