Hi viola and welcome!
To get both of your Sims to go someplace together, go to the Map View and click the location you want to go. You'll get an option to go that location alone or go with someone. If you click go with someone, you'll get a window that has the names of the Sims you can go with. Just click the name you want and click ok. They'll go together.
As for playing another Sim, you can start as many games as you want from the main menu. You can have your second Sim start fresh in Sunset Valley or Riverview, just like your original Sims did. However, if you want to play your second Sim in the same game, you'll have to use the Edit Town function on the menu and select "Change Active Household" on the left side. This lets you play another Sim in the same game, but your first family will continue on without you. They'll age and live their lives on their own. When you switch back to them, there's no telling what you'll find! Also, you'll lose all of the promised wishes when you switch to different Sim.
I hope this is helpful to you. If you have more questions, just let us know!
