You can pass as many 128 bit SSE intrinsic parameters as you like under x64. The x64 ABI was designed with these types in mind.
From the MSDN documentation:
__m128 types, arrays and strings are never passed by immediate value but rather a pointer is passed to memory allocated by the caller. Structs/unions of size 8, 16, 32, or 64 bits and __m64 are passed as if they were integers of the same size. Structs/unions other than these sizes are passed as a pointer to memory allocated by the caller. For these aggregate types passed as a pointer (including __m128), the caller-allocated temporary memory will be 16-byte aligned.