Does multible shinobi stack effects?

[gollum]

Yes. Its in MTW that spy effects don't stack.

Its most effective if you do port hoping with agents (as you hit provinces that have no army in them and usually no counter agents too), but i don't use port hoping/raiding (and even transporting in original v1.12) as the AI doesn't know the option exists.

Shinobis can be really effective in stirring trouble in provinces that are partially held. In this way you can either use them to get loyalty rebellons (if you had the said provinces for logn time) or to slow enemy advance to your territories. They also can cacth enemy agents that allows them to valor up pretty quickly and be even more effective.