THe reason, I think, for the two receivers in the ATS is almost all of our VHF, those with dual watch is the primary watch channel is still 16. Meaning if you have your vhf set for instance to , in my area 12 or 11, AND are using Dual watch, when a signal is heard by the vhf on 16 it will immediately switch to 16 meaning any radio traffic on 12 or 11 will be cut off, blocked and missed. In the busy summer time the dual watch is virtually useless. The volume of 16 traffic means the radio will almost never be on 12 or 11.
Thus the requirement for a separate vhf. VTS cannot afford that kind of operation.
I have and use two separate vhfs and have done now for quite a few years. Each radio has its own separate external speaker separated quite distinctly and without any fuss I know which radio is which. Before I did that, the speakers, if the two radios were blathering I could not tell which was which thus missed messages of some importance.
Previously I used my handheld with an ear bud and again I knew which was which.
When transiting Vancouver Harbour it helped me to many times stay out of the way and wait out First and Second narrows closures when tankers or other traffic was moving through.