Depends on the connection. Assuming it's tcp, then yes uploading uses up some of your download. And by some, I mean very little. You are basically sending back ACK packets, which are trivial in size. However, depending on how much you are uploading ( and what you are uploading ), it can add up.
I have Verizon's FiOS (Fiber Optic), which is 15MB download / 6MB, the latecy on FiOS is very good, I can upload a large .ISO to someone through FTP and have my download barely affected, meaning if I'm uploading at full 6Mbps, it'll sheer 1-1.5MB of my download VIA speedtest.net