There is no such animal as a "Pres" commission versus a "RegF" commission.
All commissioning scroll read the same: a person is "hereby appointed an Officer in Her Majesty's Canadian Armed Forces". So you can only have one scroll at a time and it is valid unless you elect to hand it in. There are no reference in the scroll to the component you are in.
This said, you can have rank and seniority dates that are different in the PRes or RegF. For instance (in the old days - don't know about today) A Reserve Major with three year seniority could take a two year class "C" posting, during which, according to his posting message, he was grant the (RegF) rank of captain with three years seniority, so that someplace mid-way in his posting he became "eligible" for promotion to major in the Reg F. Happened to my wife: She got promoted to Lieutenant twice (once res, once reg), which caused some confusion when the time for promotion to lieutenant-commander came up.
That is also the reason (single commission) why a recently promoted reserve officer that decides to join the RegF may be sent back to do training with his "peer" group of officer cadets, but will do so as a second lieutenant. No one in the CF, other than through the disciplinary process, can take away a commission.
That is why I don't understand the people here that claim to have been commissioned twice. If you go from the Pres to the RegF, you cannot be brought down lower than 2LT. What should happen is that your file should reflect that, until you graduated with your peers, you were a reserve 2LT and, upon graduation, were granted the Reg F rank of 2LT with same seniority as your peer. This should be done by message, not a new scroll). And if you left the forces for a while between the two, your file should still reflect your commissioning, unless you elected to turn in your commission.