Comment: the use of a split interconnect affects amp loading.
Fair enough. Maybe I should have mentioned that when I was first alternating A vs B in stereo I was using a single pair of RCA connectors and switching them along with the speaker cables, for that reason, to be proper and all. But after about 30 min of tugging on connectors I got lazy. FWIW, my impressions were similar at the beginning, just not as well honed as by the end. If I ever did this again in the future, I would still do the dual mono thing, and then maybe confirm with a few proper swaps. It's very helpful to be comparing audio memories from seconds, not minutes, ago.
The amp loading issue seems like maybe a non-issue, within reasonable limits. I've used this Bifrost for several years straight, and, depending on the amp I'm using, it may see 20-100 kOhm input impedance when sent to a single destination. The SN2 input impedance is about 50 kOhm, the Saga 10-15, the GJ about 20. It's been a long time since I had any physics, but if impedance behaves like resistance, I think I'm effectively presenting about 15-20 kOhm input impedance to the Bifrost, about as if I were sending the signal to my 20 kOhm Asgard alone. Pease correct me if I'm wrong.
One other thing is just empirically, for years now, I've often left my Bifrost RCA output connected in parallel to 3 or 4 destinations, just powering on what I want to use at any given time. For speakers that's only ever a single destination. But several times a month my family has 2 or 3 head amps running at once. None of us have ever noticed something off and nothing has caught on fire yet =). That is typically 100+100, 100+50 or 100+50+20 kOhm loads. The point being that the Bifrost seems tolerant to these kinds of situations.