How to control one joint while a second one swings?

edit

I have a model with 2 revolute joints spawn from an urdf, and a gazebo-ros plugin to interact with it.

I want to command the position of one joint, and the other should swing freely, but wish to know its position.

So I have a std::vector<gazebo::physics::JointPtr> sim_joints_; properly initialized with the sdf joints, and in the plugin Update() I read the two states like

for(unsigned int j=0; j < 2; j++)
{
    joint_position_[j] += angles::shortest_angular_distance(joint_position_[j],
                          sim_joints_[j]->GetAngle(0).Radian());
}

then publish all joint_position_'s, update the joint_position_command for joint j=1, and finally write

sim_joints_[0]->Update();
sim_joints_[1]->SetPosition(0, joint_position_command);

Joint j=1 moves according to the command, but joint j=0 does not swing. If I don't write anything, i.e. delete the SetPosition(), or if I use sim_joints_[j=0,1]->Update(), both joints swing freely.

Seems like writing the position to any joint makes the rest joints to freeze.

Am I missing something? How can achieve the desired behavior?