This paper considers the problem of planning closed-chain motion for a pair of mobile manipulators to transport a common payload in a dynamically unknown environment (i.e., an environment with moving obstacles of unknown motion). We present a novel algorithm to plan the actions of the two robots in the team, one leader and one helper, in real-time to accomplish the task while avoiding other ... [Show full abstract] obstacles in the unknown dynamic environment. Our algorithm does not assign fixed roles to the two team members, but rather it dynamically decides who should lead based on circumstances to optimize the team's performance. The planner is readily extensible to handle a team of more than two robots in tight collaboration. The approach is implemented and tested in simulated task environments, which demonstrate the planning algorithm's effectiveness and efficiency.