In theory could I double my legal stay in Poland by entering and exiting after three months on one passport and then stay in Ukraine for a couple of days and then re-enter on my 2nd passport which is clear?
Doesn't work like that - your data is scanned at the border and will be caught quite quickly. In fact, it doesn't matter - you'll get caught on the exit from Schengen because the stamps won't add up.
I can assure you that after a few trips to Ukraine over the last two weeks - every passport is scanned and details collected.