A catheter assembly configured for treatment of a vessel bifurcation. The catheter assembly includes a catheter shaft and first and second balloons. The second balloon extends radially outward relative to the first balloon when expanded. A valve arrangement controls expansion of the second balloon. Portions of the valve arrangement can be positioned at proximal and distal end portions of the catheter shaft. Typically, the first balloon is expanded followed by expansion of the second balloon. A stent having a lateral branch opening is operatively mounted to the first balloon. The second balloon is configured to extend through the lateral branch opening when the second balloon is expanded.