I have an E&O policy that covers me for my own general design work, but I often do work consulting for other Architectural firms due to my specific background. I make it clear in my contract that my work for them is covered under their E&O insurance. I have done this with (4) firms to date and not one of them batted an eye about it, and signed the contract with that exclusion. Of course, Architects are terrible businessmen, so they likely did it without getting approval from their brokers and/or legal department. But MY insurance broker was the one who suggested I do this.
My approach is that I they have the contract with the client, not me. Since I can't control things so I am not covering it under my E&O insurance, and my insurance would be through the roof if I did.
My own projects are small houses, retail shops, etc. The work I do for other Architects is typically high rise construction with many million dollar fees and construction costs. E&O insurance is based on risk and fee/construction costs. So that is how I keep my numbers down. For what it is worth I pay about $2000 a year for my E&O insurance.