As an slight extension of my project, I am trying to see if I can improve connectivity resilence by using a dual-SIM approach. Dual-SIM phones (or Tri-SIM, quad-SIM phones) are quite popular in many developing countries, and AFAIK, most are based on Mediatek chipsets. However, I am wondering if these use a single GSM module (RF baseband + controller), multiplexed for use by multiple SIM's, or they actually have as many GSM modules as the number of SIM cards.
If it is multiple modules, then I think I know already what is needed, but if it is a single module multiplexed with multiple SIMs, then I'd like to understand how this works. If someone has a high-level schematic of how this works, would be great to be able to take a look.
Edit (Feb 1, 2012): Call me crazy, but I went ahead and purchased an el-cheapo dual-SIM phone to pry it open and see inside, although thanks to the very high integration and density I wasn't hopeful of figuring out much. Sure enough, there are obviously 2 SIM's and the traces seem to lead into what is probably the module that is put under a metallic EMI shield box, without any markings. Can't see an obvious way to remove the EMI shield box, as it is not clear how it is stuck to the board. So, cannot tell for sure, but given the size of the shielding probably a single module.
Another reason why multi-SIM's might be using a single GSM RF module is because I found some phones claiming to have 4 SIMs !! Huh.
Edit (Feb 15, 2012): After quite a bit of reading around, I've come to believe that the way to use a single module with 2 (or more) SIM's is to modify the standard single-SIM firmware on the GSM module itself. This might be difficult to achieve, purely via the firmware on the application-processor (or uC in my case). The firmware on the module, exposes few different types of API's s.a. Hayes AT-command set, or native API that uses some kind of message-passing, and only provides somewhat higher-level control than what is required to deal with multiple SIMs. This means that, implementing dual (or more) SIM for my projects, using off-the-shelf GSM modules like the ones from Telit, Siemen/Benq, SimCom etc., isn't going to be easy (or possible). Will keep looking, and if someone has credible knowledge to challenge this finding, I shall be more than happy to learn about it.
Edit (Jul 25, 2012): I have come across 2 different handsets (el-cheapo bottom-bracket shenzhen Android phones), and a key difference in the technical specification of the 2 phones caught my attention. One said that while the handset supported 2 SIM cards, but only one of them could be active at a time, I believe it is the single module, dual-SIM approach. For the other handset, it said that for the 2 SIM cards supported, both could be used simultaneously, s.a. one being used for a phone call, and other being used to maintain a 3G data-connection. I see no way of being able to do it, without using 2 GSM RF modules. Of course, the second handset is more expensive -- by about $25, which can be explained by the presence of a better ARM11 processor (800MHz instead of 650MHz) and the extra GSM module, plug a slightly larger battery (just 200mAh extra). Of course, this hasn't been validated by ripping the phones apart, or any kind of schematic study.
Edit (Feb 12, 2013): Just to confirm, that on el-cheapo Android phones, which claim to be dual-SIM, there is quite certainly only 1 GSM module, because the behavior is that if you have data-session established, all call to the other SIM (incoming calls), fail, with the network operator playing the announcement that the number is not reachable. Effectively, it means that while the 2nd mobile number is "registered" to the network (in GSM sense), it is not reachable, because the handset (with the SIM corresponding to the mobile number), did not respond to network's "paging request". That happened because the only GSM module is already busy serving data-connection to the first SIM.