Say we have two finite sequences $X = (x_0,...,x_n)$ and $Y = (y_0,...,y_n)$. Is there a more or less common notation for the concatenation of these sequences, like $\sum (X,Y) = (x_0,...,x_n,y_0,...,y_n)$?
The comments suggest the following notations for the concatenation of $X$ and $Y$:
of which the first seems not to be in use for other concepts, making it especially suitable.
From there, and more:
I would avoid $X \times Y$, $XY$ or $X \cdot Y$ to not confuse it with any sort of multiplication / product.
And I would also not use $X \otimes Y$ because it is usually the tensor product. (See also here.)
Some relevant Wikipedia pages with common notations: