a.base1::start();
a.base2::start();
or if you want to use one specifically
class derived:public base1,public base2
{
public:
using base1::start;
};
a.base1::start();
a.base2::start();
or if you want to use one specifically
class derived:public base1,public base2
{
public:
using base1::start;
};