Have you not seen how meteor works? 6 BLMs cast 1 meteor. They all work together to cast 1 instance of the spell. As long as you start casting before the first BLM finishes, the spell will not resolve until everyone who started casting finishes.

Each BLM sends a ball of light into the sky, making a glyph appear and grow more and more complex with each light that joins. When the final BLM casts, the meteor falls.