I'm not familiar with the Agfachrome kit, but in E6 processing increasing the time of the first developer is akin to lowering the contrast and density of the final image. The amount of control isn't that great, so when I say lower the contrast, it's going to be a gentle lowering. Unless someone has specific feedback for you with this kit, I would say aim for the middle, develop a roll and see how it turns out. If it's slightly too dark, increase the time of the first developer, provided you are sure the exposure was correct. If it's too light decrease the time.
For capacity considerations, 1 roll of 35mm 36 exposure is equal to 1 roll of 120. For 2 rolls of 120 you should be fine without any adjustment.
Be sure to keep your temperature constant throughout the processing steps.