Hi David
I’d go for separate policies, you should get a better deal for the classic. Multi car cover might not offer sufficient discount to justify combining the two. You might also get charged a fee for adding to your existing policy unless at renewal. If your other car is 15 years old or more you might be able to put that on a combined classic policy. When it comes to policy renewal it’s easier to shop around with two policies given that loyalty counts for very little with the insurance companies.
Patrick