On Thursday Wolf assaulted one of my tac squads with multiple units. He told me that I could only overwatch against the first unit that declared it's charge against me b/c after the first unit made base with my tac squad it was engaged. This kept me from overwatching on the more squishy unit (I assume, don't really remember specifics) that was coming in second. I was talking with Steve about this last night and he told me that I can choose which unit I wish to declare overwatch attacks against b/c all close assault actions declared in the same assault phase are assumed to happen simultaneously, allowing me free reign to declare my overwatch attacks against any assaulting unit.

After reading the assault rules section, specifically looking for answers to this question, I have not been able to find any specific rule saying that a unit attempting overwatch can be locked by an assaulting unit before shooting attacks have been made, or that I in fact do have free reign to declare which unit I wish to make OW attacks against when being assaulted by multiple units. In other words, I haven't been able to find any mention of overwatch being used against multiple units assaulting a single unit.

If someone could please point me to a page number where this situation is clarified I'd appreciate it.