Suppose an initial state $A$ can transition to either state $C$ or state $D$. Suppose further that both of these two processes are rate-limited by a transition to the same intermediate state $B$, as follows:



Note that $\Delta G_{AB}&gt; \Delta G_{BC}&gt;\Delta G_{BD}$.

According to transition state theory, rates are determined by the rate-limiting step. So the rate of each process will be the same,

\begin{align}
r_{A\to C}&amp;=r_{A\to B} \\
r_{A\to D}&amp;=r_{A\to B}
\end{align}

from which it follows that the number of transitions made during some large $\Delta t$ will also be the same,

\begin{align}
N_{A\to C}&amp;=r_{A\to B}\Delta t \\
N_{A\to D}&amp;=r_{A\to B}\Delta t
\end{align}

However, if we were to consider the two pathways collectively then we would predict more transitions to be made to state $D$ than $B$ since $\Delta G_{BD}&lt;\Delta G_{BC}$.

How do we resolve this apparent discrepancy?


Question: Is it possible to keep using iPhoto in parallel with Photos?
Answer: You can keep using it, but not in parallel.

Any changes made in one app will not be reflected in the other.
Over time, things will diverge more &amp; more.

I would use one &amp; stick to it for now.
When you finally decide to go over to Photos full-time, then do a resync from your current iPhoto state, by deleting the Photos Library &amp; importing again.
Doing that will then erase any work done in Photos &amp; replace it with the latest iPhoto state