Unfortunately, the BOGO feature was not designed with a buy multiple, get multiple option in mind. You can't allow this as a mass offer because it's difficult to limit and you risk the possibility of customers being able to receive multiple discounted products when purchasing a single full-priced product.
To get around this limitation you could create multiple, more targeted offers.
- Buy one shirt, get one shirt free.
- Buy one necklace, get one bracelet free.
- Buy one pair of jeans, get a second pair at half price.