The full proposal is quite complicated and has to resolve questions around indirect branches, FAR CALLs, privilege transitions and interrupts and other details of the Intel architecture that, although mostly unused, are still necessary to produce a working operating system. The proposal also works with existing code.
To summarise, if you had wanted to introduce this feature about 40 years ago, it would have been trivial and (quite probably) implemented purely as a compiler code-gen strategy. If you want to implement it now, it is "quite fiddly".
(Actually, implementing it 40 years ago would have been a little fiddly as well. Traditionally the heap grows upwards from the bottom of a segment and the stack grows down from the top. A second stack would have to find a third "end" to grow from. Not insurmountable, but enough of a headache that you'd optimise the solution by storing return addresses and automatic variables in the same stack.)