Can Gas Ejectors be connected in series?

Yes.
Single stage Gas Ejector installations are the most common but in some applications it is necessary to connect two (or more) units in series in order to achieve a significant compression. In a two stage system for example, the discharge gases from the first stage are fed to the suction of the second stage.
In marginal gas well applications a single stage Gas Ejector is usually employed.
In flare gas recovery / elimination applications where overall compressions greater than 10 bar are required, a 2-stage GAS-JET system could be employed.