Overall, I think this is a helpful discussion begun by Michael, though I would encourage him to come back and interact a bit with some of the responses.
Although I work for Accordance, don't look at my message here as an "official response" as much as just further conversation from someone who has been an Accordance user since the 90s. One of the programmers or perhaps someone in senior administration could offer something official, although I'm not certain it's necessary.
I can say that these kinds of suggestions for more than two panes have been part of of ongoing internal (and even external) discussions. There's no conspiracy to limit the mobile experience to give favor to the legacy platforms. It's just a very different beast and takes a bit longer to get an entirely different codebase on a platform, that uses different kinds of input, up to speed with software that's been around for 25 years. And as I like to point out, Bible software in general, and certainly Accordance in particular, is much more complex and allows for many more tasks than most of the other applications on one's device--whether legacy or mobile. That is, there's a big difference in what Accordance does on the iPad compared to even something like the very polished Microsoft Word and Excel apps now available for iOS (and Android).
I, too, had OliveTree's app on my Palm device back in the day. I even had the Gramcord Greek NT from Accordance that was a joint venture with OliveTree for it. Other than the two-pane issue, I can't agree that "nothing has really changed in almost 20 years"; and I especially would take issue with "the functionality of these mobile apps seems stalled." Both of these statements seem hyperbolic. Bible software is much more capable today than those early readers 20 years ago. If I have to, I can dig my Palm Treo out of a drawer to prove it, but I really don't think that's necessary.
In the third post of this thread, Fraser listed a number of features in Accordance Mobile that would have been nearly inconceivable from a development perspective back in the days when we were all using Bible software on our Palm, WindowsCE, and Newton devices. Although mobile Bible apps may not have progressed as quickly as we all might've liked, I believe the limit to two panes has primarily been dictated by screen size and processing power until recently. Now we have bigger screens. My iPad Pro is 12.9"--a larger screen than Apple's 11" MacBook. And the processor in it is capable of editing multiple 4K video streams at the same time (something I would definitely not recommend trying on the Core m MacBook).
I expect we will see software, including Accordance Mobile, continuing to leverage both larger screens and more powerful processors to add advanced features that we take for granted now in Windows or Mac. The Affinity Photo app for the iPad is the first of what I would consider the next stage for mobile computing. It's coming--not as quickly as we might like--but it will get here.
Again, as preface to my next few sentences, I remind anyone reading this that I'm not a programmer. When we talk about a tabbed interface in iOS, which is what would be needed in Accordance Mobile for something like Constructs or to perform Infer searches, I don't know if Apple provides this in their toolkit or if it would have to be created from scratch. I mentioned the other day in another thread that I see tabs in Safari for iOS as well as GoodReader. The ones in GoodReader are not very "good" (pun intended), and I assume they are homegrown. I notice that even programs like MS Word or Excel (or the Apple equivalent of Pages and Numbers) for iOS only allow ONE document to be open at once. That's pretty frustrating when you need to put content side by side; yes, there are ways around it, but no one likes having to do that. Just like you can put two Bible applications side by side on an iPad Pro to accommodate four panes at a time (I've done it!), but we really don't want to have to do that. But if you compare Accordance Mobile to MS Word or Excel, at least we don't limit you to one title in your Accordance library! In fact, with popup Instant Details and User Note editing, we've essentially allowed for three panes, if but for limited instances.
Regardless, no one at Accordance (and I guess I have to assume for the other guys too) is sitting around thinking we've done everything we can do for the software and the platform. The ability to do more has only truly presented itself in the last couple of years.
[One more thing--my name was mentioned in some of the posts above in regard to my previous statements about legacy vs. mobile platforms (the whole truck vs. car thing). Although I'm tempted to comment on that here, I'll save that for another time, perhaps in a different discussion.]