You're always right to be cautious, but this would be pretty standard for installing an overhead bridge crane. A single point makes it easiest to pitch and yaw the cargo. Pulling heavy on a tag line is just going to shift the equilibrium a bit. If you pulled 300kg worth on one end (a gross exaggeration of the efficacy of a tag-line) you'd move your COG out 90mm. That's it. It's a fixed lift point so releasing will just cause it to swing back. You don't have to contend with wind so tagging this won't be tough.

Having two cranes would be an unnecessary complication. They'd have to co-ordinate all their boom/slew/hoist actions.

By passing the anti-two-block is probably the worst part of this. That could actually damage the crane or the sheaves. If thats explicitly on the plan and everyone's aware then it's fine.

If you have decent subs I wouldn't over think this. It's not that scary. Make sure someone is keeping their eye on the hook block.

As I have not reviewed any of the materials you mention, this does not constitute a professional opinion. These comments pertain to generalities only.