The ideal break-in is lots of varying engine speeds and loads, but it is not a great idea to subject the engine to heavy continual loads early on as this leads to localized overheating of parts that have not yet established working clearances. I'd back off a bit from doing sustained high speeds and avoid long sustained grades when you're two-up, but otherwise, ride normally.
I'd also suggest not paying a whole lot of attention to Yamaha's very conservative engine break-in RPM recommendations, other than avoiding sustained high RPMs. Religiously adhering to these limits two-up would might result in lugging the engine which is much worse than using a little extra RPM. The bike will let you know when it is happy dealing with the load.
These things have a lot of leeway and it would be difficult to abuse a motor enough during break-in to lead to failure, but a little bit of extra care can result in an engine that runs a bit better or lasts a little longer.
- Mark
