By the way there are a couple of phones available now that support 4G or 3G on the primary SIM slot and 3G on the secondary SIM slot
simultaneously. For instance the OnePlus 3 does so which I've been owning for more than two months now. But despite this improved capability it is dual stand-by only which indeed means that I cannot receive calls on the other SIM while I'm having a phone call.
There's a corresponding thread on XDA devs:
http://forum.xda-developers.com/andr...pable-t3370632