Two fiscal years open simultaneously?

Based on everything I've read for Sage 300, I have to finish posting all adjusting entries and close my fiscal year before I can open a new year.

Suppose my year end is December 31, 2017. It will take a month or more to finish posting all the entries for 2017. Meanwhile I can't post anything to the GL in 2018 until I open up the new fiscal year, but doing this will effectively close all P&L accounts to retained earnings for 2017.

Is there a way to open 2018 and start working in it before I finish 2017 - essentially having two fiscal years open at the same time?

If not, how is this supposed to work?