new: https://neo900.org/stuff/papers/simsw-draft.pdf on http://neo900.org/resources
dual-SIM design - gamma draft

[edit] I noticed the purpose was not as clear as it should be, so here's the explanation ([edit2] and a better intro in the pdf):
Neo900 (tentatively) comes with two SIM trays, one as usual (from N900) under battery, and an externally accessible one embedded into the spacer frame which is a slot-in type

• For modem you can swap cards in tray A and tray B electronically, but that's no Dual SIM Dual Standby where both SIM are active concurrently.
  The PLS8-rev3 modem (not released by Gemalto yet) allegedly comes with 2 SIM interfaces so the modem might do DSDS.
• For NFC the NFC-MCU has two SWP interfaces and it's up to user loading a firmware to the MCU to make use of both of them.
• For the CPU: it can use both SIM trays as smartcard reader slots, but the modem can't have access to either of the SIMs during times where CPU does smartcard reader

[edit2] updated: draft->release version