I'm somewhat perplexed by the fact that I got a SELCAL light on the "transmitting" VHF system. You would think only the receiving system would react to the incoming tones. I'll have to try this on another aircraft type to verify the results.
Without the camera, I tried the HF systems from #1 to #2 and vice-versa. I did not receive a SELCAL response from either system. Off the top of my head and without looking it up, I'm pretty sure the non-transmitting HF R/T is "removed" from the antenna system while the other R/T is transmitting. Possibly through the coupler or some type of contactor. This of course, prevents damage to the receiving side of the non-transmitting R/T, but also would prevent it from receiving SELCAL tones in my test situation.
I'm pretty sure two aircraft would be required for HF SELCAL checks using the phone app.