Hi,
Welcome to ehealthforum.
It is not impossible to get pregnant even though you have PCOS. There can be occasional egg release from the ovaries depending on hormonal levels which can result in pregnancy. The second period, if lighter and shorter than your first period, would suggest possibility of implantation bleed.
Another possibility of having second time bleeding in same cycle could be breakthrough bleeding, which is possible due to prolonged hormonal contraceptive use.
Consider visiting your doctor/gynecologist if you do not get your periods at the expected period time (normal period time) for examination and further investigations to look for the underlying cause.
Take care.