Now we have so many mic types, you can try the ones that let you get close without proximity effect and sing into them close. The amount of guitar will be virtually zero. The guitar mic will get the vocal reduced to a degree by a big mic in the way, but there will be some spill.
let’s assume the aim is to be able to provide processing and effects to one and not the other, or maybe you want to comp the vocal track, or even pitch shift it? With a close mic you can do that, and when you blend them together, you’ve fixed it. With two more distant mics, perhaps to avoid proximity problems, this kind of vocal work is very difficult. If you just repeat the recordings till your fingers and voice are perfect, balance and blend is all you need.