I believe it is doable with reflector lib, but you should first google a bit for what methods are available, in ordr to first determine if the device where your app is installed has dual-sim capability
Dim i As Intent
i.Initialize(i.ACTION_CALL, "tel:" & telNumber.Replace("#", "%23"))
'this is where all the magic is!
i.putExtra("phone_type", 1) 'For sim 1
'or
i.putExtra("phone_type", 5) 'For sim 2
StartActivity(i)