The flame spread and flame interactions between multiple solid materials such as cellulose paper and PMMA were studied. The flame interactions were observed by changing the geometric arrangements of solids which includes six types of configurations such as: single solid, two parallel solids, four parallel solids, L-shaped solids, ㄇ-shaped solids, and enclosure type solids. At different separation distance and solid width, flame shape, flame size and flame interactions were recorded and flame spreading rates, flame and solid temperatures were measured and compared. The effects of flame interactions on the flame spreading rates were then analyzed. The results shows, for most of the cellulose papers and PMMA, the flames spread faster as the solid widths become wider. For thermally thick material such as PMMA, the flame spreading rate is lower than that of thermally-thin cellulose. For parallel solids, the flame spread behaviors are quite different than the single flame due to the flame interactions between solid fuels. The flame spread rate increases as the separation distance decreases because of the flow channeling effect and radiation interactions. The flame spread rate reaches the maximum at an intermediate separation distance and then decreases as the separation distance becomes smaller due to the flow resistance and limited thermal expansion. The highest flame spread rate is the enclosure type of solids, followed by ㄇ-shaped solids, four parallel solids, two parallel solids, L-shaped solids and single solid.