You don't have to enter the event twice, you just need to direct the S Planner to show events from multiple calendars. To do this, open your S Planner, then click the Options in the upper right corner (three horizontal lines).
Choose Settings
On the left panel, you should see all Gmail accounts linked to your calendar. The top option on the left panel is View Settings. Tap this.
Under View Settings on the right panel, choose Calendars >
Put a check mark into all calendars you want to see displayed in the S Planner. If your second account is not included, click Add Account at the bottom and set it up.
OK to back out.
When you add a new event, make sure that you add it to one of those 2 Gmail calendars (NOT to the S Planner "My Calendar") and it will display on the S Planner, and on both of your GMail calendars.
Click to add new event (+ sign at top of screen)
First item to select in the template is Calendar. Use the drop-down to select the appropriate calendar. (Defaults to "My Calendar" but I *think* once you choose a different one, that different calendar is sticky until you change it again.)
That should resolve things for you!
