ok so this isn't difficult to do, but it requires a few steps which may look long typed out, but it's pretty simple.
for this example, i'm making the FC12 the first FC and the FC6 the second FC. that means the FC12 has the FASlink XLR cable connected directly to the Axe, and the FC6 is using the power adaptor and 2nd XLR cable. you can do it either way, but when we assign things to specific FC numbers, it matters - you just have to choose the correct one depending what order you have it plugged it.
another concept to be aware of is that all FCs share the same Layouts. FC6 doesn't have its own 9 Layouts and FC12 doesn't have its own 9 Layouts for a total of 18 - the same 9 Layouts are shared. so Layout 1 on the FC6 is the exact same Layout 1 on the FC12. this isn't an issue, because we'll be using different Layout numbers, but do be aware they are exactly the same.
if you enjoy the default Layouts, i'm going to teach this in a way that keeps most of them as is. ideally, you could/should make your own Layouts that make more sense for what you need. for example, just because the default Layout 2 is "Scenes" doesn't mean they have to be only Scenes. i put Looper controls on mine as well. keeping things separated can help to teach the functions, but for actual performance, it's usually better to put what you need on a Layout so you don't need to change often.
i'm also going to show this using Axe-Edit primarily, rather than the hardware, as that's easier to show.
so here's how to do it while keeping most Default Layouts intact.
so FC12 is FC#1 and FC6 is FC#2. i've reloaded the FC12 Default Layouts, as that's our "main" controller with all the actual things to switch.
because we're using that Default, Layout 1 has Preset switches, Layout 2 has Scene switches, and so on. so we don't need to make any changes to the first 5 Layouts for this example - they already do what we want.
we are going to change Layout 6. it defaults to Per-Preset switch placeholders, but we're going to use that as our FC6 Layout for selecting the other Layouts on the FC12.
so pull up Layout 6 in the FC Controllers section of Axe-Edit. change the "Show As" area to FC6, since we're working on changing it for the FC6.
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then click the EDIT menu near LAYOUTS and choose Reset Layout:
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this will clear out all switches on this Layout only, and that's what we want.
now with the blank Layout, we can add what we need to make the FC6 choose what the FC12 displays. i'm going to put things in a certain place, but of course you can put things wherever you want.
Switch 1 will make the FC12 show Presets - which is Layout 1. so first we are going to set the Tap function of Switch 1 to Layout, Select, Layout 6, View 1; and we're setting the Mini-Display Label to Custom, and typing "Presets" as the custom name.
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we need to choose Layout 6 because we actually don't want this switch to do anything. we will already be on Layout 6 on the FC6, and we don't want anything to change on the FC6 itself. so this setting really makes it "do nothing." so whatever Layout you use for the FC6, make sure the switch is set to change to that same Layout.
now the part that will make the FC12 change: click "Edit Links" near Layout Links on the right side of Axe-Edit:
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that pops up a menu. remember our FC12 is FC #1 - connected directly to the FASlink XLR cable. so we just change the FC#1 area for TAP to say Layout 1, which is our Preset Layout.
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after this is changed, click the X on the top right to close. and that's it. now when you are in Layout 6 on the FC6, pressing and releasing Switch 1 on the FC6 will make the FC12 change to the Preset Layout. note that this change will happen when you
release the switch. that's just how Layout Links work.
from here, just repeat the steps for the other Layouts. let's set Switch 2 to call our Scenes Layout, which happens to be Layout 2 by default.
you can set Switch 2 manually, or Copy and Paste Switch 1 to Switch 2, but the Layout Link won't copy - you'll have to set that up again. also remember to change the Custom Name to "Scenes."
but most important, remember that Switch 2 should also be set to Layout, Select,
Layout 6, View 1. remember, we're making this switch "do nothing" when you press it. only the Layout Link is causing anything to change. we do not want the FC6 to change Layouts at all. so any switches that tell the FC12 what to do will be Select Layout 6.
the Layout Link for Switch 2 will be FC#1 - Layout 2. again, this is because Layout 2 happens to have Scene switches setup.
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you can also change the Color of these FC6 switches to whatever you want.
so just keep repeating this for each switch, and setup the Layout Link to whatever you want the FC12 to change to.
once that is all setup, you need to make one more change so it all boots to the correct Layout when you start.
in Axe-Edit in the FC Controllers page, click "Devices" near the top right. change FC#2 (the FC6) Startup Layout to 6, and also change Current Layout to 6 as well. this means when the FC6 turns on, it will start on Layout 6, which is important because that's where the switches that we just setup live.
FC#1's Startup doesn't really matter too much because you'll be changing the FC12's Layouts with the FC6 anyway.
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and that's it.
remember on the FC6, it's the Layout Link that is making the FC12 change.
you can also put anything else like Tap Tempo and Tuner on that same page of the FC6 if you have switches you didn't use. there's really no limit, just whatever makes sense to you.