Yes and yes. Juice Plus is basically fruits and vegetables, with a few other natural ingredients. So it is both safe and beneficial to take during pregnancy and breastfeeding. You should also continue to take your prenatal vitamins during pregnancy and breastfeeding to ensure you are getting 100% of each and every vitamin (since JP is a natural mix of fruits and veggies, the manufacturer cannot precisely quantify the amounts of specific vitamins in the product).
Taking both together is best. You will not overdose on any vitamins this way. After all, you can’t eat too many fruits and vegetables, can you?