We do multiple attacker exercises right from the beginning.
The first exercise is part of the warm up: all students move on a small part of the mat, actively seaching partners for a short contact, then moving on. Crashes are not allowed. What your're learning is first to see the gaps and free spaces were you can go and not to concentrate on the people and second to be the one to choose whom you deal with. This way, the surprise is on your side.
The second and third execises are part of the randori at the end of the training: two uke attack a single nage with all kinds of attacks allowed. And: two uke attack a single nage who has his/her eyes closed (no strikes or kicks, of course).
Training with two uke is very helpful to keep nage from thinking about what to do and planning the next technique because nage always has to keep moving.
And with closed eyes, you have to rely on your feeling of where the force and the flow goes.
And it is great fun, of course.
Hope that helps,