Yes - I use the insurance disc from my daily as that's the policy that you will be driving under too.
Just check with your existing insurer - I was with 123.ie (RSA) previously and they insisted that the other car you're driving had to have a valid insurance policy in place by the owner (no idea why) but FBD said it didn't have to be insured in the other 'owners' name as it was them covering the liability.
Current Daily: 2008 Lexus GS300 Sport F/L
Current: E39 525i M Sport (Titan Silver)
Previous: E34 520i SE (Orient Blue)
Yes, most insurance companies are very vague about the above. Is it just to deter people from doing this? Funnily enough, on my own policy, in the Product Document, it states that I have Comprehensive cover when I drive other cars , providing the car in question is not owned by me and is 3,000CC or lower.
Current:
'06 530d Silver Grey, '04 325Ci Orient Blau
Previous:
'03 M3 Silver Grey
'02 325Ci Imola Red II
'98 318iS Cosmos Schwartz