SMPS for powering 2 loads
SMPSs are, in principle, constant power devices. So, for your setup you would have 240 watts available, and a load of (24 x 5) + (12 x 8) or 216 watts. Like you say, the math works out.

What this ignores is efficiency. SMPs are not perfectly efficient, and for a homebrew unit I'd be surprised if you get 90% efficiency. This boosts your load requirement to exactly 240 watts. Any further decrease in efficiency will increase your load requirements further. 

Also note that, at these current levels, circuit layout will be critical, as will choice of components (getting your inductors right, for instance, may give you more heartache than you realize). 

As a beginner you are definitely jumping into the deep end of the pool, so don't expect things to work the first (or even the third or fourth) time.