You've encountered an issue present in the 2012.1.215 version, which has already been fixed. Please, download our 2012 Q1 Service Pack 1, released earlier this week, and give it a try - the chart should behave as expected in the described scenario.
About sharing an axis in WPF please check our help topic on multiple axis here in our online help system.
the Telerik team
Sharpen your .NET Ninja skills
! Attend Q1 webinar week and get a chance to win a license! Book your seat now >>