Dual-SIM phones are a great idea, useful in many situations.
For example, when I travel internationally, I buy a local SIM to avoid exorbitant international roaming charges. Since most of my existing phones are limited to a single SIM, I have to remove my India SIM and keep it somewhere safe, making sure I don’t lose it. And then I no longer get calls and SMSs sent to my India number.
National roaming is also a problem in India. When I moved from one state to another, I had to get a local SIM in the new state. When I did, I also wanted to keep my older number for a while, both because I had some balance in it, and because people were calling me on that number. I couldn’t do that with a single SIM phone.
So, dual-SIM phones are great. When I got one, I couldn’t get it to connect to 3G . It was connecting to mobile data only at EDGE or GPRS speed. I realised after some confusion and Google searches and trial-and-error that the phone supported 3G on only the first SIM card. This is irritating and arbitrary and forces you to swap SIM cards. Dual-SIM phones should support everything they do on the first SIM on the second as well . You shouldn’t have to bother about which slot you use any more than you worry about which USB port you plug a pen drive into.
And if you do need to insert a new SIM or swap SIMs, you should be able to do that without powering down the phone. It’s just an unnecessary hassle, especially when you’re tired and jet lag.
You should also be able to put one SIM into silent mode (including turning off vibrations). When I travel to the US, I may want to keep my India number active, but I don’t want to be woken up at night (in the US) by calls from every Tom, Dick and Harry during the day in India. So, you should be able to put one SIM into silent mode, including turning off vibration.
Dual-SIM phones should be able to simultaneously use data on both SIMs for higher speed. In most countries that lack superfast LTE networks, we’re stick with 3G at best. The phone should make different TCP connections over different SIM cards. This will increase bandwidth and reduce latency.
Android lets you set a data limit on your phone. Dual-SIM phones should let you set a separate limit for each SIM card, and turn off data on that card when you reach the limit .
Dual-SIM phones should also intelligently decide which SIM to use for calls and SMSs. If one SIM is roaming internationally, the phone should avoid using that SIM for outgoing calls and SMSs.
In summary, making a dual-SIM phone is easy. But these can be irritating to use and generally not have a great user experience. OEMs should focus on making not just any dual-SIM phone, but one with a great user experience, taking all the above factors into account.
 It should ideally be LTE on both SIMs. In India, LTE, where available, costs no more than 3G, so the phone should not be the bottleneck.
 If the phone supports 3G on one SIM and only EDGE or GPRS on the other one, maybe background data (such as email syncs) can happen on the slower SIM, as long as its within quota, to preserve the faster one for use by the user.