EVM
Stack too deep
It will be possible eventually, but I don't think we need to keep this issue open. You could also do things like:
bytes memory a = abi.encodePacked(1,2,3,4,5,6,7);
bytes memory b = abi.encodePacked(8,9,10,11,12,13,14,15);
`keccak256(abi.encodePacked(a, b));
EIP663