Kinda. IANAL, but here’s my understanding: If you’re explicitly dual-licensing and publish the proprietary license then contributions can be assumed to also follow the same dual licensing. You’d need to be extremely careful with writing the proprietary license though, since your business is now using non-employee proprietary code.
If you write “the copyright holder may choose to allow an entity to use this work”, then you do actually need permission from every contributor. If you write “this work may be copied, modified and redistributed freely by Blah enterprises” now the business cannot be sold without losing access (or possibly have it’s name changed). If you write “Neshura may freely copy, modify and redistribute this” then you can’t be fired or move jobs without the company losing access.
You can also never ever change this license, since every contributor needs to agree. So if a mistake is made when writing it you’re just fucked.
On the other hand with a CLA that transfers copyright ownership you don’t need to dual-license at all since everything already belongs to the business. Much less risky.
🤔