One happy pair, they just wed,
And their love, they will spread.
Saw them once, they pass by;
With joyance, cheered by nation,
A single word, thought I,
Can itself, sum the situation.Years have passed, love is slain,
Now they go, different ways.
They divorced, but again,
A single word, fits the case.
The second word, like the first,
Two core letters, just reversed.
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$\begingroup$ Are both the words nouns? $\endgroup$ – Rand al'Thor Dec 14 '16 at 13:01
I think two words for the two verses may be (but this reply may be missing clues)
UNITED and UNTIED
One happy pair, they just wed, And their love, they will spread.
The first verse describing united - they wed is united
Saw them once, they pass by; With joyance, cheered by nation,
May be the United Nations
A single word, thought I, Can itself, sum the situation.
The first word being united
Years have passed, love is slain,
Now they go, different ways.
Untied so may go different ways
They divorced, but again, A single word, fits the case.
Untied for divorced
The second word, like the first, Two core letters, just reversed.
United and Untied have the two core (middle) letters reversed