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There is always a possibility you will not be admitted. The visa waiver programme is intended for infrequent, short visits
Immigration may find two entrances at maximum length separated by such a short space of time suspicious. I suspect you would not be denied entry, but you may be subject to extra questioning on your second entry which could take a few hours and be quite unpleasant. You will need to be ready to show that you have a plan to spend your time, that you are a tourist, that you have sufficient funds to sustain yourself, and you do not intend to defeat any immigration rules such as by overstaying or trying to work. It is up to you to prove this, not for US CBP to prove the opposite.
It may be better to obtain a proper B2 six month tourist visa instead.