Correct got the same problem over here.
One solution is to use two different modems, with complete different chipsets.
When using two of the same IVM just uses the first one in the list.
Sadly enough two different modems is also not an solution for me, because i need to have three lines, so two modems for sure.
This is a problem in ivm not in winodws.
E.g. you can use both modems exactly the same, on the same time, buth not in IVM.