It used to be that rustic full size bed frame were automatically sized individually, but that’s not the case anymore. The builders are now stacking larger beds to save space while still giving the sleepers a lot of space. Plans for complete over full berths (as they are frequently called) are available, but most should be purchased. A good compromise is to review some of the free plans for two bunk beds and full size single beds, then combine the best qualities of both.
You can build two separate beds instead of a single unit, but the construction is more complex, since you must build two headers and two stirrups that can be interchanged. This requires centering and half-inch drilling holes in the top of each pole of the headboard corner. And in the bottom of each corner post stirrup. Next, use the two headers with the lower bunk and the two headboards with the top bunk, and insert half-inch diameter plugs into the holes to keep the beds together.
You will have to leave at least two feet of head space for your sleepers, regardless of whether they have superior capacity or in the frame. Platform beds leave more space for this than the mattress and box spring combinations do.