Can I Use A Thick Or Double Layer Of Underlayment?

You are able to use thick underlayment. The thickest that an underlayment is made is 3mm (1/8”). You should not use double underlayment because it will create too much cushion and padding between your stable subfloor and your floating floor. When there is too much padding, you will experience weak and unstable joints, which may cause the planks to separate.