I have used 2 & 3 USB modems in Linux a few times. Not sure about windows but best bet would be to install the usb drivers for each modem and NOT the whole software for dialup given by the mfrs.
Then set up windows normal dialup thing and select the virtual com port assigned for each modem (see in device manager - you can see in com ports that each usb has got a different numbered com port).
There is software available to combine various internet connections into one - search on google. I just used different routing for different modems in linux but have seen some How-To on combining them (sort of load balancing - so that one request goes through one modem and another goes through another one.)