They both do the same thing so you're going to have conflicts when trying to use both at the same time. I would not recommend doing this at all.
The biggest reason to use something other than BitLocker for full-system encryption is if the competing product provides something vitally important that BitLocker simply doesn't do yet. Microsoft changes the feature set for its core programs very slowly, with Windows being on the slowest refresh cycle. Microsoft doesn't want to change things so quickly that any attempts to build on top of Windows as a deployment platform are negated. That leaves plenty of room for third-party software makers to leap in and fill the gap, as many have done.
Same reason you don't want to have 2 different antiviruses. Always causes conflicts.