Your Insurance company can temporarily transfer cover to the temp car - just call them and find out what their procedure is. You'll need to supply them with the make, model, licence plate etc on the day. Usualy it's done over the phone.
If you are dropping your car off early you can do a temporary sub the day before to take effect from say 830 am as some insurance cos and brokers are not open(as i found out when i tried to phone fbd the other morning)
You can ask that there is an overlap as far as I know, just so that you have a bit of leeway timewise ie both cars covered from 8am to 10am and same for collection time.
Rental car would be a TAD temporary additional vehicle on policy.