It not quite as simple as that I am afraid. By using MHSRP (multigroup HSRP) then you can achieve crude load balancing but one router will always be active and another be standby.
To have a true active active relationship between 2 routers where both are forwarders then it is better to look at GLBP rather than HSRP.
By implementing GLBP you are only dealing with your traffic one way though, i.e. from your network into the SP cloud, and still need to consider how your service provider will route traffic back to your head office. You will need to speak to them and tell them what you are going to do and ask them to implement load balancing on their side as well.