I wonder since when did Insurance Companies became involved in deciding regulation? Apart from using it to avoid payments...
I would suggest having a look at the CAA Safety Pilot Information Sheet and particularly the extract below:
WHAT IS AN OPERATIONAL SAFETY PILOT LIMITATION (OSL)?
This limitation is added to a medical certificate when a pilot is considered to be at increased risk of incapacitation compared to his/her peer group. The holder of the medical certificate is precluded from solo flying and always has to fly with a safety pilot
Now put yourself in the place of a regulator being asked the question, can 2 safety pilots fly together?
As neither here are allowed to fly solo, and always have to fly with a safety pilot, how can either one comply with this regulation in the case of the other becoming incapacitated?
IMO, therefore, the answer has to be No.
The correct course of action must be to ask the CAA and, if by some chance they allow it, keep a written record of their answer in case of later problems with Insurers.