Yep, I fully agree as far as I know the hardware design of FP2/3.
Most mobiles are made this way, but of course there are others available too.
I found this very interesting read about the differences and a work around for not missing calls when one SIM card is actively in use for calling.
As the phone has only one transceiver built in (DSDS specification 
)
→ DSDA two transceivers built in → two call/data connections simultaneously.
I wondered about this behavior too just some weeks ago and thought at first my phone was malfunctioning as I did not put any focus on this behavior up to then. When phoning I prefer to concentrate on my calling partner not necessarily doing other things with my phone.
But I found a simple work around if internet connectivity anyway was needed, using wifi if available should help here.
Generally spoken it is funny that even the chipset of FP2 is capable of being designed for a DSDA mobile.
and the one in FP3 could be DSDV.
But as mentioned in the read most (big) manufacturers don’t want this and there would be a slightly more expensive design and maintenance needed.