What's the purpose of a cold water expansion vessel?
An expansion tank is needed on a section of closed line that is subject to variable pressure. The only sane cause I can discern for having two tanks in close proximity relates to multiple devices with multiple check valves or an undersized expansion tank. Whether that tank is before or after the water heater is usually irrelevant. The job is to take up liquid expansion between a closed faucet and a supply line check valve.

It may be the case that your water for radiators is separated from your drinking water. Two distinct systems with a check valve to prevent radiator water from draining back could explain a second tank.