Dreaming of a twin often brings up duality -- two sides of yourself, or two paths you're weighing, mirrored back so you can see them side by side. A twin who feels like a rival may reflect inner conflict between competing desires, while a supportive twin can point to integration, a sense that both sides of you are finally working together. The dream usually asks which choice you're avoiding.
You can address Mercury on the empty signs while the Moon can be addressed on both positions indistinctively. - Picatrix (Andalusia, ~1000.AD)