No. If you are holding onto your car then your car policy is using that bonus. You then need to hope you can find an insurer that will allow you to "mirror" that bonus or allow some level of introductory bonus on the back of. Which are few and far between these days. So you can only use one NCB on one car. You can't have 6 policies and use the one bonus you have on all 6.
My late brother worked for a well known courier company. When I say worked for, I should say was subcontractor. He found it very, very, difficult to get insurance for his work and at that time , aprox 10 years ago, there was only one or maybe 2 insurance companies giving insurance for courier work. He did get it but was paying a very big amount and he was a very experienced and claims free driver.