In NDIS mode the internet connection is seen by the operating system as another network interface. So I think this would fix the problem.
On the other hand if you connect multiple usb modems on single usb port using a typical usb hub it won't work due to current limitation in usb port (max 500mA). Since each modem draw 500mA.(refer E153_product_description.pdf)
In this case you will have to use a AC powered USB hub to connect the modems. Where the power will be provided by an external power supply.
Guys, this thread is well over 4 years old so I'm going to close it, but do feel free to start fresh threads if any of you are having any issues with Linux.