I also did both checkrides on the same day, which is fairly common here. Since you are familiar with the 172 I would talk to the examiner and see if you could do your initial commercial in the Seminole and then jump in the 172 for your single engine add-on. This is what I did and it saved me some money not having to rent a complex single. Basically, your initial checkride is the only one that requires a complex aircraft (for the landings.)
I'm not sure if you are instrument rated, but if you are, I would also try and fit in some approaches during your multi-engine checkride so that you can also add on multi instrument privileges all in the same day.
Just remember, 2 checkrides equals 2 fees if you go with a DE. So prepare yourself for a long and expensive day.