In 2006, I ordered a Custom Mixer to routing and connect the STICK signal. I was very pleased with him. Many yeras later I sold it on this forum. The buyer from US now sells it.

viewtopic.php?f=5&t=13458

Cheap, useful, small, and sounds well with Stickup.

another cool option i can recommend is the EHX Switchblade Pro
(NOT the old one switchblade plus which is just passive A/B/Y switch)

you can plug melody side to main input and control its level with "dry level"

bass side to return A control its level with "a Level"
and still have free input retrun B - for..mmm guitar/stick with mono option

mixing them through the output jack

That unit is specifically an amp switcher from what I see: not an ABY switch that can go either way where one can also switch between two guitars. This thread made me look at what I’ve got buried away in boxes. I do have a cheap Live Wire Solutions ABY pedal that works both ways, but when I just ran my newer Stick with the Stickup it does suck the volume and a little tone out of the signal. I also have one of those plastic adaptor stereo to mono adapter plug, that actually sounds better but does have a slight tone variation just like comparing a good cable to a cheap one. If you go with an adapter plug look for something high quality. Otherwise get a proper ABY box that can run two guitars in an amp, or many DI boxes will also do the same. Some other good suggestions by others in this thread as well.

These ABY gadgets can destroy amps in the hands of guitar players that don’t know what they are doing with them. When I checked out the reviews on the Live Wire I have some people were asking if they can run it between the amp head and two cabinets.

...wait a minute - I think that's what I've been using, the Radial ABY and it's been working great for me.

Now I'm going to have to double check that - because I vividly remember going to Sam Ash a couple of years back looking for just that - a combiner type box - and the kid from the pro audio/keyboard section was in the guitar area, he said hold on, ran over and ran back with one of those and I asked the same thing, I said isn't it one way ?? and he said trust me it will work, and it did.

*disclaimer - I only used it with my Warr Phalanx which is set up like a blaster beam....

But by doing a quick Google search, because I'm too lazy to go look for the pedal - I'm pretty sure this is it: https://www.radialeng.com/product/bigshot-aby

If it's not, I'll update this post.

Mark said he had the other one, the Twin City. https://www.radialeng.com/product/twin-city

I’ve also been able to combine stereo to mono using an EBS as well as a Sansamp DI pedals as well in the past, even though the manuals said nothing about being able to do that. I’m glad this subject came up for the opposite reason, as what I have in storage can be dangerous as an amp switcher if it would ever go bad. It’s better to use what you guys have.

Yep...they make several different versions.
I used the Passive one live for a split to two amps and then upgraded to the Twin City which only flows one way.

Running dual paths on Helix with identical settings works really well.

Stereo to Mono adapters cut the signal but a good 6-9db or so.
I haven't checked to see how the actual tone is affected but I would expect an Impedance screwup.

I wonder if a Passive A/B/Y would be any different.

Do you know how the combined pickup level and tone are affected by adding the Resistors?

Found something....not cheap but I generally like Radial

https://www.sweetwater.com/store/detail/MixBlender--radial-mix-blender-dual-instrument-buffer-mixer-and-fx-loop-interface?mrkgadid=1669547280&mrkgcl=28&mrkgen=gdsa&mrkgbflag=0&mrkgcat=studio&recording&&acctid=21700000001645388&dskeywordid=39700048796761137&lid=39700048796761137&ds_s_kwgid=58700005374436735&device=c&network=g&matchtype=b&locationid=9013121&creative=452849193071&targetid=dsa-879330602807&campaignid=6730319008&gclsrc=aw.ds&&mrkgcl=28&mrkgadid=1669547280&rkg_id=0&campaigntype=dsa&campaign=aaDSA&adgroup=1669547280:DSA%20-%20Product%20-%20Mixers&placement=google&adpos=&creative=452849193071&device=c&matchtype=b&network=g&gclid=CjwKCAiAudD_BRBXEiwAudakXwt-J6VdzparD2IEUCaRWoVJY-ZgJp0BlsNBoYyJ4dH4H5B-JjwOHRoCLXAQAvD_BwE&gclsrc=aw.ds

Sorry for the late answer... yes, there's a bit of a volume drop. However, for me it's quite ok volume-wise and tone-wise.