Long term, a mid-engined platform is going to happen. JDN wants it, and so does the Corvette team. Financially and logistically, it makes sense to produce such a vehicle and have both Corvette and Cadillac share the same basic underpinnings.
That said, could they get both? Could it be both exclusive to Cadillac AND shared by both brands? In short, Yes.
What I could foresee is that
Cadillac would have the platform exclusively for a few years, to launch a mid-engined offering and gain wide recognition for their halo sports car. Later, after Caddy has had the product/platform to "itself" for a couple of years, I could see Corvette jumping on board with an
additional Corvette branded product (i.e. a mid-engined Corvette to supplement -- and not replace -- the current front-engined Corvette).
A few thoughts on this:
*Corvette is valuable nameplate that, in my humble opinion, must expand beyond it's traditional role as a single-product offering. It is also a revenue source that has been completely under-realized by GM. I've
written about this before in a few places on GMI, but the bottom line is if GM wants to make the most of their assets, Corvette should become it's own brand with an expanded range. In doing so, the company could not only better square-off against dedicated sports-car rivals at Porsche (or Maserati, Aston Martin, etc), but also allow GM to reap huge profits and attract new customers. As such, adding a mid-engine Corvette (and creating a stand-alone Corvette brand that continues a loose association with Chevrolet) can add huge economic realities to any mid-engine platform program.
Taking this a step or two further, any stand alone Corvette brand could also share a select number of platforms with Cadillac. As Cadillac pushes into new territories, a limited Corvette range could help make any planned Cadillac products more feasible and likely to be built if the R&D costs are spread to another vehicle program. If JDN wants to truly flush out Caddy's lineup beyond core-products, and into niches with limited volume (think of "low volume" Caddy niche products to compete against offerings like the Audi TT, BMW Z4, SLK/SLC, etc), then making a solid business case is vital and necessary.
*JDN wants his own Audi R8 and he knows that something this sophisticated can't be shared by one division alone for cost reasons. If he's copying Audi's playbook, then he has to look to VW AG as a whole to figure out how to make this work.
At VW AG, the need for an R8 rival was premised on using the Lambo Gallardo's platform. In allowing Lambo to introduce the product first, Lambo gained recognition for it, and it stood out as an "in-house" product. This allowed Lambo to push the brand into a new segment while also "protecting the perception" that the R8 was based on a Lambo --- and not vice-versa. That's huge since it meant that not only was the R8 a more credible sports car, but that the Lambo used a dedicated platform which was later lent to a corporate cousin.
That last bit is incredibly important and it's the way forward in this scenario for Caddy and Corvette. If Caddy gets the platform first, and Corvette jumps on board later, then it looks like Caddy lent it's assets to Corvette, rather than a vice-versa. Think about the perception they're trying to cultivate at Cadillac and it's easy to see why they have to have their product stand alone first. If the reverse were true, then you'd have a very expensive Cadillac sports car based on a "lower-end" Corvette. The perception among rich customers would that that Cadillac merely "tarted up" a Corvette for Caddy-duty. This could potentially harm that image they're trying to create. It's not a knock against the Corvette, but merely a perception that they're trying to nurture as it seeks to chase rich, well-off customers.
Much like VW, this has also played out elsewhere. Take a look at the Giorgio platform at Alfa Romeo; it will spawn a raft of products for Alfa (and Maserati), but it's also supposed to be eventually shared with Dodge and other FCA products so as to make financial sense. In allowing Alfa to use it "exclusively" for a few years, it helps to create the perception that future Dodge's are "based on a pricey Alfa platform", rather than basing a slew of Alfas on a "ho-hum" Dodge platform.