Convergence of Collatz sequences

I don't know if this has been studied, but I can partly explain it.

Write $f(n)$ for the Collatz rule. Consider the binary representation of $n$. If it ends in a 0-bit followed by exactly $t$ 1-bits with $t>1$, i.e. $n=a2^{t+1}+2^t-1$ with $a\ge 0$, then $$ \begin{align} f(f(n-1)) & = f(a2^t+2^{t-1}-1) & = (3a+1)2^t+2^{t-1}-2 \\ f(f(n)) & = f((3a+1)2^{t+1}+2^t-2) & = (3a+1)2^t+2^{t-1}-1 \end{align} $$ That is, $f(f(n))-1=f(f(n-1))$ and $f(f(n))$ ends in a 0-bit followed by exactly $(t-1)$ 1-bits.

If we write $\langle a,t \rangle$ for $a2^{t+1}+2^t-1$ then we have the rule $f(f(\langle a,t \rangle)) = \langle 3a+1,t-1 \rangle$ when $t>1$. Note that this changes the parity of both parts $a\to 3a+1$ and $t\to t-1$. Using this notation your example starting with 351 goes: $$ 351=\langle 5,5 \rangle \to 527=\langle 16,4 \rangle \to \langle 49,3 \rangle \to \langle 148,2 \rangle \to \langle 445,1 \rangle = 1781 $$ where each arrow is applying $f$ twice.

Finally, if $n=8k+5$ then $$ \begin{align} f(f(f(n-1))) & = f(2k+1) & = 6k+4 \\ f(f(f(n))) & = f(f(24k+16)) & = 6k+4 \end{align} $$ so the sequences starting with $n$ and $n-1$ converge after 3 steps.

Now $n=8k+5=\langle 2k+1,1\rangle$, so if we start with $\langle a_1,t \rangle \to \langle a_2,t-1 \rangle \to \cdots$ and arrive at $\langle a_t,1 \rangle$ with $a_t$ odd, then the series converges after 3 more $f$-steps. Since the $a_i$ have alternating parity, this will happen if we start with any $a_1\equiv t \pmod{2}$.

In summary: For any $a\equiv t \pmod{2}$ with $a\ge 0$ and $t>1$ let $n=a2^{t+1}+2^t-1$. Then $f^c(n)=f^c(n-1)$ where $c=2(t-1)+3$ (and this is the smallest value of $c$ for which they match).

This is sufficient to generate examples of converging sequences. e.g. let $a=1001,t=3$ then $n=16023$ and the sequences go: $$ \begin{array}{llll} 16022,8011, & 24034, 12017, & 36052, 18026, 9013, & 27040,\ldots \\ 16023,48070, & 24035, 72106, & 36053, 108160, 54080, & 27040,\ldots \end{array} $$

However, there are other possibilities, as your 237,238 example shows. That particular case can be explained by observing that if $n=4k+2$ then $f^3(n)=3k+2=f^3(n-1)+1$. Here $f^3(238)=179=\langle 22,2\rangle$. In general we can get to $\langle 3m+1,2 \rangle = f^3(32m+14)$. You should be able to generate other rules like this by trying to solve $f^c(n)=f^c(n-1)+1$ for small $c$.

I think the simplest answer for this is that this doesn't just happen with number's n and n+1, it happens with any two numbers you choose (assuming the conjecture is true). You can see this easily if you look at a Collatz Tree. The sequence for every number is obtained by just following the number down towards the root of the tree. If the conjecture is true, then every 2 numbers has at least some of their paths in common.