My understanding of the eligibility criteria is that you may elect to join AEA while working under a "contract" in 2 separate productions
My understanding is that you can be join Equity working on any one production under contract, though some contracts specify minimun requirements for the "Stage Manager" position, as opposed to the "Assistant Stage Manager" position.
(I know a number of SMs who got into Equity by being hired as SM on a TYA contract, specifically for TheatreWorks/USA.)
This one of the reasons Off Broadway has become flooded with SMs in recent years -- many producers will hire a newbie ASM and get them their card, knowing that the young stage manager is less likely to be familar with the details and intent of contract as well as less likely to raise a fuss about non-compensated additional duties.
As a tangental note: every year I see postings and/or hear stories about "producers" who offer the reward of EMC (Equity Membership Candidate) points as an inducement for working on Showcase Code productions. There are NO EMC POINTS FOR SHOWCASE and currently no EMC points available
at all Off Broadway.
(Does anyone know if the theatres that operate in NYC under a LORT contract [e.g. Roundabout] are permitted to offer EMC?)