Hi ,
Genrally it can be posible due bandwidth configuration on tunnel interface but ther is no harm in monitoring both the interface,it is genarlly a benfit only for you as if tunnel goes down it will raise an alarm also for the same.
For exact monitoring for tunnel interface i would suggest you to check - VPNTTG (VPN Tunnel Traffic Grapher).
Advantage of VPNTTG over other SNMP based monitoring softwares is following: Other (commonly used) softwares are working with static OID numbers, i.e. whenever tunnel disconnects and reconnects, it gets assigned a new OID number. This means that the historical data, gathered on the connection, is lost each time. However, VPNTTG works with VPN peers IP address and it stores for each VPN tunnel historical monitoring data into the Database.
Hope that helps out your query !!
If helpful do rate the valauble post.
Regards
Ganesh.H