well I'm no expert, still getting up to speed on BMS. You can find some details at pp. 82 and following of the BMS manual and there are threads at the BMS forum like this one:

http://www.benchmarksims.org/forum/showthread.php?7137-TTS-Trouble&highlight=amraam

Basically, it works in a similar fashion to TWS RAID mode in Jane's F/A-18. It appears to work in both RWS and TWS.

The procedure I use is the following:

1. slew radar cursor over 1st target and "designate" (HOTAS TMS up);
2. repeat for 2nd target;
3. you can then "step" from one target to the other (HOTAS TMS right). When you are in TWS, you will notice a white cross next to the currently designated target;
4. when in range, fire AIM-120 at first target, "step" to 2nd target and fire another AIM-120.

Your radar should track both targets and update the missiles appropriately. You can practice using the BMS AIM-120 training mission.

Thanks for the tutorial, it really helped me.