The most common and most likely answer to this crossword clue is the 9 letter word LUNISOLAR
I've seen this crossword clue in The Times
L
U
N
I
S
O
L
A
R
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The straight part
We know that "Concerning two bodies jointly" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case concerning two bodies jointly = lunisolar.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does LUNISOLAR mean?
LUNISOLAR adjective- (of a calendar) Based on both the lunar month and the solar year.
- Pertaining to or caused by both the moon and the sun.