I agree it will feel super nice to go from the front screen to the dual screen and have nothing "move". And similarly it will feel "mildly infuriating" if it doesn't do that.
I also think that I (personally) don't need a front screen and would love to have whatever cost savings or improvements would come if there weren't one. This last sentence is of course a me thing, but I mention it because I think that if people start on the front screen and then open the Duo, they may just want the full screen app or something else entirely. I'm not sure the use case of being in your banking app device folded, deciding midway through flow to open it, and wanting to be in the same UI is really what I want. The part I question is what's going on that step 2 came about—I think it's either you want to go into more depth on it or you're just opening the device the way you would had you not been in your banking app.
The case where I open it to find the app using half the screen and then I have to press something or drag something to make it full screen is not that great (and isn't great today with current iPad window resizing). The best case I can think of for wanting the app in the same size format when I open it is for is something like multitasking, I guess we'll see how that looks or feels and how you get the second app open (or whether it's very common use case anyway).
In general, I've long felt the folding tech is for tablet use cases not for phone use cases. I think Apple's foray (finally) into it more or less validates that. Even though there will be a meaningful number of people buying it to replace their phone, I think this is largely because their phone use is already tablet use case-y (for entertainment especially). I assume many people will also buy it as a(n expensive) tablet replacement in addition to their phone, which is perhaps related to the timing of iPhone Handoff (same SIM on two phones) too.