Change the orientation of Arrow of xtwocell in the 2cell option for xy

Change the orientation of Arrow of xtwocell in the 2cell option for xy

I'm trying to make a diagram similar to this (from wikipedia)

Except that I want to the double arrow $\eta$ two be pointing from node
$B$ toward the middle of arrow $X$ (it is, after all, a natural
transformation from $RF$ to $X$).
I'm trying to use the \xtwocell command of the 2cell package for xy. In
subsection 3 (Excursions) of section 25.4 (extra options) of chapter 25
(two-cell) on page 47 of the xyrefer manual, it is stated
\xtwocell[⟨hop⟩]{⟨displace⟩}... connects to
the ⟨pos⟩ displaced by ⟨displace⟩ from the
relative cell location specified by ⟨hop⟩. The displacement
can be any string of valid XY-pic commands, but they must be enclosed
within a group {...}. When the cell location is the target, a null
grouping {} must be given. When used with the <\omit> nudge, such
excursions allow a labelled Arrow to be placed anywhere within an XY-pic
diagram; furthermore the Arrow can be oriented to point in any direction.
Of course the manual gives no examples and makes no further comment about
how to change the orientation of the Arrow (in this manual, capitalized
"Arrow" refers to the double arrow between arrows, i.e. the 2-cell).
What does he mean by "any string of valid XY-pic commands"? What may the
displacement argument for \xtwocell be? Any experiments I tried (eg
<1>,dr,[0,1]) typing in the curly braces just ended up in the $A$ node
text, so not being interpreted as a displacement or position or
orientation. How do I change the orientation of the Arrow? How do I
achieve the "orientation in any direction" that was promised me?
Here is my simple markup
\xymatrix{
A\ar[d]\ar[r]\xtwocell[0,1]{}\omit{<2>} & B\\
C\ar@{.>}[ur] &
}
which gives the following image

I want to change the direction that double arrow is pointing, point it to
the corner object $C$, or reverse it. I can shift it by changing the
number in \omit{<2>}, but how to change the orientation?
(Google showed me that the same question is asked here on the xy mailing
list, but never answered)
See also my question at How to label the source/target object in the
middle of compositemaps from 2cell package of xy? for another attempted
method at this diagram.