Being rated in two types is actually pretty normal in many airlines. It is called MFF or mixed fleet flying. However a certain familiarity between both types should exist, therefore it is mainly airbus fleets where it is done. There are excemptions for simulator checks. We use that currently on our A320/A330 fleet. The normal number of simulator events is done, every 6 months at least two days, however they change between types each time.
There was also a certified MFF scheme for boeing 737 and 757 or 767 for some time, however we never operated the larger boeings. But there is currently a lot of work being done to develop a 737/787 MFF system to the same rules as on the airbus fleet although the differences between both types are rather large.
Without a certified MFF system one can hold two active typeratings for AOC work, however the normal simulator events and checks have to be done for each type, in an airline environment that would lead to four simulator events per year. I would think the same is true for dual class ratings.