Low-carbon transitions require joint efforts from electricity grid and transport network, where electric vehicles (EVs) play a key role. Particularly, EVs can reduce the carbon emissions of transport networks through eco-routing while providing the carbon intensity service for power networks via vehicle-to-grid technique. Distinguishing from previous research that focused on EV routing and scheduling problems separately, this paper studies their coordinated effect with the objective of carbon emission reduction on both sides. To solve this problem, we propose a multi-agent reinforcement learning method that does not rely on prior knowledge of the system and can adapt to various uncertainties and dynamics. The proposed method learns a hierarchical structure for the mutually exclusive discrete routing and continuous scheduling decisions via a hybrid policy. Extensive case studies based on a virtual 7-node 10-edge transport and 15-bus power network as well as a coupled real-world central London transport and 33-bus power network are developed to demonstrate the effectiveness of the proposed MARL method on reducing carbon emissions in transport network and providing carbon intensity service in power network.