Try this procedure... it worked for me to put an app on my wife's RT user account:
Although it may look like it ... it does NOT download another copy of the software but it does install the app.
good luck ...
1. Log into your second account on your device.
2. Open up the Store.
3. Open up Settings.
4. Click on Your Account.
5. Click on Change user.
6. Log in with a Microsoft account that has already purchased apps from the store.
7. Swipe down from the top and select Your Apps.
8. Select an install any of the apps that you had already purchased with that account.