That is impossible to answer without knowing what PSU you have, what GC you bought (exact make and model for both preferably) and what's in the rest of your system, i.e. what your CPU is.

What I can tell you is that the adapter is for people whose PSU does not support 150 Watts on a single 6 pin connector, so they have to combine two to connect them to the 8 pin connector on the GPU. If the PSU supports it, it usually has a dual 6/8 pin connector that allows both uses.