You got an answer to the specifics, I'd like to just add that in general, a lot of what is considered safe diving practices boils down to preventing, catching and defanging mistakes. Your suggested plan may be fine when everything goes as planned, including the failures you're planning for.
However, there are lessons (written in blood) about what happens when people jump in breathing from a different regulator than they think they are, or with one tank that's empty when it was supposed to be full, or closed when it should have been open, etc. The best practice setups guard against these mistakes, or include steps that ensure they are discovered early and in a non-critical stage of the dive.
One of the more fervent pony adherents on the forum has a story from recently when he splashed and was mistakenly breathing from the pony instead of the main tank, and was very surprised and close to panic when finding himself out of gas very early in the dive. This is a classic back-mounted-pony mistake that isn't a thing with, for example, sidemount, backmounted doubles, or a larger single tank with a Y valve.
Your further idea of using the pony to extend the dive time by riding the reserve limit precisely in both your tanks also makes me twitchy. Not that it won't work when it works, but that it reduces the headroom for mistakes, getting distracted, etc. I like leaving myself lots of space to **** up in and still be fine (and then not **** up, ideally).
