Here are the answers to the puzzles in my last post. All of these involve words formed by the odd and even letters of each of the examples.
TRIENNIALLY is the longest English word in which all the odd-numbered letters spell a word (TINILY) and the even ones do, too (RENAL). In this case, both spell the words forward, but other examples do not. FURRINESS, BALLOONED, and FLEETNESS are the next longest ones with this property.
FRIENDS has this same trait, but with a twist. The two odd/even words, FINS and RED have letters in alphabetical order. FINS in forward order, RED in reversed.
SPOILED has the same trait except its words SOLD and PIE both have letters in reverse alphabetical order.
SINNING is interesting in that its internal words SNIG and INN have the alphabetical orderings reversed/forward, but it also has the property that SNIG (which is a British slang term) also spells a word backward (GINS).
The longest word I’ve found where the two words are both spelled backward and in alphabetical order is UTOPIA (IOU, APT) if you count IOU as a word. If you don’t care about the alphabetical order, the longest words I’ve found with both odd/even words spelled in reverse order is THICKEST (SKIT, TECH) and GIANTISM (STAG, MINI) although there were dozens of words of this length with one word forward and one backward. I may have overlooked other examples.