Since I can't see & interact with your view I can't say for certain, but so far in simple testing I've been unable to drag and drop multiple resources onto multiple tasks at once. The Task multi-select is lost when I multi-select the Resources and vice versa.
Here's a possible solution.
Multi-select your resources that you want to place on your multiple tasks, right click then select Modify.
Click into category and type 'a'.
Now, with the instructions in the previous post multi-select the Tasks, Right Click Assignments.
Click the Category header to sort your 'a's on top.
Multi-select them and click Assign.
After they're assigned, go back and remove the 'a' before you Save.
HTH