Quoting POSIX Shell Command Language,
A <backslash> that is not quoted shall preserve the literal value of the following character, with the exception of a <newline>. If a <newline> follows the <backslash>, the shell shall interpret this as line continuation. The <backslash> and <newline> shall be removed before splitting the input into tokens. Since the escaped <newline> is removed entirely from the input and is not replaced by any white space, it cannot serve as a token separator.
This means that, in your first example, what the shell actually executes is
echo "some text" "some more text"
which is the simple command
echo followed by two arguments, concatenated using a space character when printed to standard output.
In your second example, what the shell actually executes is
text="some text" "some more text"
where the first line is interpreted as the simple command
some more text (a single token, including the space characters) preceded by the variable assignment
text="some text"; then,
echo $text is executed.
To produce the same result as the first one, your second snippet may be changed into
text="some text "\
"some more text"
Note, also, the double quotes in
echo "$text", needed to prevent the shell from applying word splitting and filename generation to the expansion of
$text (it makes no difference with your sample strings, but it would if they contained whitespace character sequences other than a single space and/or globbing characters).