if P1 and P2 are two project names keep both the projects open. go to gantt chart view amd split and have predecessor and succesor in the bottom pane. go to ID (bottom pane) in P1. if task 5 in P1 is successor to task 3 (or any other task no)of P2 then write in ID P2\3 - click OK you will find...
well u can have the s curve in msp itself .go to view-resource graph. right click -choose cumulative cost.then choose bar styles and then choose filtered resources total costs and choose line. click ok there u have ur s curve with cumulative costs shown on vertical and monthly cost on horizontal...