I just hadn't got here yet. Disappointment ends now
The rule of thumb is: use whatever gives you the better signal. Since you already have a huge dish, point that at one or the other, then add a second dish for the other slot. All other things being equal, you will always get better signal with a single dish pointed directly to a single target than with any multi-LNB dish. Sounds like you like to play with nonstandard configurations (gee where have I heard THAT before, oh yeah, my wife) so here goes.
Since you specifically mention the DP21, you need one per tuner, and you'd have to split the outputs of each LNBF if you have too few outputs to match the number of tuners you need.
Logically speaking though, there is probably something you want on 77 (have you looked at thelist yet to see if you'll be missing anything?) so that would require a 3rd dish and a DP34, DPP33 or DPP44; or the aforementioned 1000.4.
There are many factors to consider: signal strength; size of area to hold proposed dish farm; spousal approval; etc.
Thanks Pepper. I agree with your analysis of the situation. I just like to hear other peoples opinions. And I am a do it myself'er. I have 61.5, 110/119 and I could swing the 61.5 to 129, but I will have to get by some los issues so I thought that it would be easier just to convert my twin lnb to a single and turn it to 72.5.