I had successfully used the twin sim adapter earlier in nokia 3100 without any problems, at that time i had to restart my phone to switch between the sims.
I had heard that there's a code to switch the sims without restart the phone,
I think you can't use two sims simultaneously,
but if your sim activated by missed call alert you can get those missed no's when you switch over the particular sim again.
I like this method coz I've installed some softwares like call recorder, black list etc in my phone, so I can use those appz for both the sims with the same N70.
I've also activated missed call alert so I'd know the missed no's later.