You can re-use the chaga chunks or powder for a second large brew if desired, but the potency will be reduced. It is advisable to increase the steep time with each re-use as the strength of chaga reduces consecutively.
If you have a big chunk of chaga, it is better to best to break it into smaller units of 1 inch each to make chaga tea. You will have to use a hatchet or a very sturdy knife. However, it is best to powder the mushroom chunks.