If your work involved 20,000 miles a year then sure.
But if, like most people, you do 10k a year or less any benefit in terms of MPG rarely, if ever, covers the cost of buying a second vehicle.
I guess what you really want is something thats fast, handles really well, is great fun in the twistys, can do cool power slides, is fun to throw around for when you want to have fun, looks brilliant and you can take to a track day and then for when you just want to go to work/go on a trip, you want something that's reasonably economical, loaded with kit, supremely comfortable, reliable and safe.
That'll be a 330i/530i then