TLDR - I would stick to your single JC5 and enjoy the music.
I'm currently running a JC5 and A21 with an external Bryston 10B electronic crossover, JC2 Preamp and JC3+ Phono. I have the JC5 in use with Mids and Highs and the A21 for Lows below 300hz. I did this because I built some charge-coupled Internal crossovers for the mids/highs of some big vintage JBL 4343 speakers, so I require biamp with an external crossover if I want to use them. Otherwise I would stick with the simplicity of a single JC5. Every extra device and interconnect is asking for more hassle, cost and source of noise or point of failure. Money and energy better spent on sources and vinyl ;)
The dual mono JC5 and BP2 crossover deliver the blackest blacks I've ever heard when there is no input or if the music is quiet. Something I never really understood to be desirable until I experienced it and was like, wow, so that's what they're talking about. Cross-talk of the 2 mono block amps in 1 chassis aren't a problem that needs to be solved, IMO. But some people simply want the absolute theoretical best which separate monos would in theory be more separated because they are. Monos of different topology, lower power and strained transformers would make sense, but with the JC5 it's simply not an issue. There are JC1's available if you must monoblock.
The point from another poster above about leaning treble direction is one to pay attention to. When you have highly accurate top end from speakers like KEF and B&W (which I used for years) you have to be very careful about some amps, gear and sources getting to brittle/shrill/harsh sounding up top which can bring on listening fatigue in my case. For me, speakers that use titanium tweeters are a no-go with many common amps that others like in different systems.
This top end fatigue and problems are precisely why I went Parasound over Mc. The JC5 was designed by John Curl to be more tube-like without tube-problems and obviously gobs of additional power to drive anything. You'd think you would be missing detail but that is simply not the case. The JC5 is truly a unicorn and paired with the JC2 preamp, you have something truly special.
I would only consider a second JC5 if you are trying to stay in class A, which you probably are already staying in at your listening levels and setup. That amp delivers 12 Watts of pure class A, which I doubt you're getting out of. Then in an exceptional execution of class AB, you have 400W into 8ohms and is stable down to 2ohms, so there is more power than you will ever need on tap.
I would only consider Mc if you simply must have those blue meters or are going after a known-good, specific setup. Nothing wrong with that and McIntosh makes amazing gear, I just outgrew the need to have the meters and am very happy with my Parasound JC system.