I did something similar. As a Marriott Platinum, I asked Starwood to match and will easily make platinum at both this year. I find it very useful. There are many places that I travel where a Westin is 1 mile from the customer but the closest Marriott is 10 miles away. Likewise, when I visit my headquarters, there is a Marriott right across the street which is my top choice. I like having the flexibility of choosing the convenient hotel without giving up the benefits. It's also nice to have the points on both when it comes time for vacations. Marriott doesn't have a single hotel in Africa so I'm saving my SPG points for a nice African vacation while using Marriott for fun weekends around the US.

As always, YMMV...