We're doing the same. I re-wrote our code over the weekend from sending solely through Mandrill, to splitting our traffic 50/50 between SendGrid and SparkPost. I'm going to implement a similar strategy for some other external services we rely on and I've open sourced the Go code [1].
This also has the benefit that if one service goes down, we can automatically fallback to the other with (hopefully) no downtime. Most email providers have a limit to what you can send until you're established with them, so using another provider as a standard backup isn't feasible, as suddenly sending 1,000's of emails a day will see the account get suspended pretty quickly.
Plus, if one of them goes out of business or changes their terms with little notice, like Mandrill, we should have a bit more time to work around it as at least one service will work.
This also has the benefit that if one service goes down, we can automatically fallback to the other with (hopefully) no downtime. Most email providers have a limit to what you can send until you're established with them, so using another provider as a standard backup isn't feasible, as suddenly sending 1,000's of emails a day will see the account get suspended pretty quickly.
Plus, if one of them goes out of business or changes their terms with little notice, like Mandrill, we should have a bit more time to work around it as at least one service will work.
[1]: https://github.com/dchesterton/go-service