In regards to your query, you will not be to connect to two devices in this way. The intended way to use this is to be able to pause on one device and play on the other.
I hope this helps!
May 11, 2021
Jun 15, 2018
Bluetooth (BT) is a communications protocol which allows two devices to talk/communicate with each other. Both ends implement/perform the communication by making calls to a set of computer routines sometimes referred to as an application programming interface or API. These API's along with the data they manipulate is in BT terminology referred to as a profile. This is NOT something exposed to you as a user unless you were planning on designing custom 'firmware' on the QC35ii which is clearly not possible since the QC35 firmware is proprietary to Bose.
So you will have to merely use the functionality provided by the firmware that resides in the QC35ii to communicate with your smart devices and computers. Makes no difference what other non Bose headphones and speakers they support for BT (even if other headphones do what you want) since again BT depends on the implementation of both sides. Bose has 'by design' in all their BT 4.x products created a communication environment that restricts the incoming audio to one communication type/profile at a time. You can either be listening to stereo streaming (A2DS profile) or phone/voice over IP (HFP profile) at any given time. HFP will trump. So when a VOIP application takes over your computer/smart device, the stereo stream will pause until HFP is done and yields back to A2DS.
Again, this is how all Bose BT headphone and speakers work. It is consistent across their product line and considered a 'feature'. So it will not likely change unless for some reason they change all their devices (again unlikely).