A dedicated synth unit is overkill, but an Axe FX Ultra for just the synth block is as it should be? I get that it's probably mostly about not having to spend more money, just pointing it out
For what it's worth, I am running my Kemper in the FX loop of a Helix. The thought was that the Helix has a really great work flow, and I liked the modularity of it, so the idea was to use the Kemper as a dedicated "amp block" with built in stereo effects. It's working really great, however, I will be downsizing to just one modeller, and then hook that up to a desktop mixer and some dedicated rack FX. The problem I see with running a modeller in the loop of another is the stacking of latency, conversions and noise. Each modeller has an internal latency of around 1-5ms, depending on how much stuff you have in your preset. On the Kemper I think the max is 3.7ms? Or whatever value the "fixed latency" setting is set to. So, it's not like I'm noticing it or feeling it, it's not 30ms, it's not like I can sense any delay. It doesn't really bother me. But the signal my guitar outputs is now being converted to digital going into the Helix, then converted to analog going out of the Helix, then to digital going into the Kemper, then back to analog going out of the Kemper, and finally digital when going back into the Helix. I have Line 6 Powercabs so those are hooked up with AES, so no conversion there, but the Powercabs are digital and have their own internal latency too. If I also connected to them with analog, that would be yet another round of conversions.
So anyway, for all intents and purposes, it works flawlessly. I'm not noticing anything play feel wise. Some times I use a L6 G10 Relay going into the Helix, and monitor on wireless Siberia 840 headphones. So I'm 100% wireless, going through two modellers, and I still can't really say I'm noticing anything that's messing me up. It feels fine to me. But on the conceptual level, the idea of all those conversions didn't use to bother me, and now they do. One thing is the added latency, another thing is the fact that A/D conversion adds hiss to a signal, so the more times you do it, the more times the near-inaudible hiss gets layered on top of itself. At a certain point it becomes actual audible noise. All of this makes me worry about the "signal integrity", so my new line of thinking is I want to output a mostly "pure" guitar signal into a mostly transparent mixer. That way, the "dry" part of my tone will be my guitar -> kemper input -> kemper output -> analog mixer. One A/D conversion and one D/A conversion + the internal Kemper latency Vs . three A/D, two D/A + internal Kemper latency + internal Helix latency.
You either will start worrying about stuff like that down the road or you won't, so maybe that can inform your choice a little
Yes, or just run kemper Left and axe Right. There are NO rules.
With two Kempers, you can do that thanks to the "Fixed latency" setting. With two analogue amps you could do the same. But with two modellers, their internal latency will vary, and you could have phase issues. I have done this with the Helix and Kemper and it sounded fine, but the problems introduced by phase misalignment are above my expertise to address, so I generally try to avoid having them in the first place.