Multiple PPPoE connections through a LAN switch

Generally speaking, that kind of setup might work, depending on PPPoE client and server implementation.

When client wants to connect to DSL access concentrator, it first sends PPPoE Active Discovery Initiation broadcast packet, which in case you have multiple DSL modems connected to LAN (L2) switch will reach all your service providers. The packet contains required server/service name, and although it will reach all connected concentrators, they can decide (based on packet data) if they will respond with PPPoE Active Discovery Offer packet, which in return contains server information (MAC address, concentrator/service name).

Once the client has recived offer(s), if the PPPoE client implementation supports it, it could pick only the exact one it intended to contact in the first place and continue with the rest of the stages, communicating only with the one concentarator (based on receives MAC address).