Is it possible to use both the GPUs in a MacBook Pro at the same time?
It depends what you mean by:


  use them both at the same time


So, Apple's usage of two GPUs is based on using one or the other, not both. Meaning as a user, you cannot "use them both at the same time."

Now, if you're talking about using them both as a programmer, well then anything is possible, and you could probably target both GPUs using OpenCl.

Most applications written for OSX though will target GPUs using OpenCL, which is Apple's abstraction of the GPU. This means apps will call on the GPU when needed, and because Apple's implementation is one GPU at a time (not both), then most apps will only use one, not both.

NOTE:
This is in regards to using multiple GPU chipsets, not multiple core within a chipset. Obviously OSX and OpenCL can use multiple cores.