Hi,litouch,
We are facing the same issue.Although my environment is something different from you.Actually i need two router act as hub service for separate user,but i only got one 7206vxr, so it must act as hub for separate user at the same time.
Then i create two tunnel on the 7206,and i also facing "invalid spi errors".Only one tunnel can work normally.
I couldn't fixed the problem yet.