Mineral scaling is a common problem in geothermal power facilities. This scale deposition is usually caused by changes in the geothermal fluid temperature or composition and chemistry. Precipitation of minerals can limit fluid flow within the steam field equipment, reducing plant efficiency and increasing maintenance costs. A layer of scaling is commonly found in most parts of the geothermal steam gathering system. However, significant amounts of deposited mineral scales are usually observed in the pipelines and vessels that handle silica super-saturated brines. This work focuses on a less common scaling in geothermal two-phase pipelines, when fluids are usually high enthalpy and under-saturated with respect to amorphous silica. The scaling is mainly caused by mixing and changes in steam fraction at the two-phase pipelines, when several wells share the same pipeline header to transport the fluid to the downstream facility. Two case studies are discussed in this work reporting mineral scaling due to the mixing of incompatible geothermal fluids within two-phase headers. The first case investigates silica deposition caused by the mixing of a steam-dominated well fluid with fluid from two water-dominated wells. The scale deposition is caused by the small amount of brine entrained in the steam-dominated well fluid, which reacts upon mixing with the fluid from the other water-dominated wells, causing massive localised scaling at the mixing points. The second case investigates iron sulfide and silica deposition caused by the mixing of a low-pH, high-silica fluid with neutral-pH fluid. Header blockage can result in an increase in wellhead pressures causing production from some wells to collapse, at the same time decreasing the flow of geothermal fluids to the separators. The scaling requires regular cleaning to return the pipelines to full flow capacity. Recommended engineering solutions are given for both cases, for possible site implementation.