One DbContext and multiple database, EF code first

This is my first question, and I know I should search before asking anything, I am sure that I have done search but I didn't find appropriate information.

I am using code-first approach to implement my Context and my Models, So I have a simple Context like:

[DbConfigurationType(typeof(MySql.Data.Entity.MySqlEFConfiguration))]
public partial class MultipleContext : DbContext
{
    public MariaDBContext(string connection) : base(connection)
    {
        //Database.SetInitializer<MultipleDBContext>(new MariaDbInitializer());
    }
    public virtual DbSet<Test> Tests { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Test>().ToTable("test")
            .HasKey(e => e.ID);
    }
}

and my model:

public partial class Test
{
    public int ID { get; set; }

    public string Name { get; set; }

    public string Family { get; set; }
}

and I have two connectionstrings like below:

<connectionStrings> 
    <add name="MariaDBContext" connectionString="server=127.0.0.1;user id=root;password=xx;database=sb1" providerName="MySql.Data.MySqlClient" />
    <add name="SqlDBContext" connectionString="Data Source=localhost;Integrated Security=SSPI;Initial Catalog=db1" providerName="System.Data.SqlClient" />
  </connectionStrings>

and my EF config :

<entityFramework>
        <defaultConnectionFactory type="System.Data.Entity.Infrastructure.LocalDbConnectionFactory, EntityFramework">
            <parameters>
                <parameter value="mssqllocaldb" />
            </parameters>
        </defaultConnectionFactory>
        <providers>
            <provider invariantName="MySql.Data.MySqlClient" type="MySql.Data.MySqlClient.MySqlProviderServices, MySql.Data.Entity.EF6, Version=6.9.8.0, Culture=neutral, PublicKeyToken=c5687fc88969c44d" />
        </providers>
    </entityFramework>

I want to write to different Db just by changing ConnectionString like:

MultipleDBContext context = new MultipleDBContext(System.Configuration.ConfigurationManager.ConnectionStrings["MariaDBContext"].ToString());
var xx = context.Tests.Where(x => x.ID > 0).ToList();
context.Tests.Add(new Test()
{
    Name = "name",
    Family = ""
});
context.SaveChanges();
xx = context.Tests.Where(x => x.ID > 0).ToList();
//Use sql connection

MultipleDBContext sqlContext = new MultipleDBContext (System.Configuration.ConfigurationManager.ConnectionStrings["SqlDBContext"].ToString());
var sqlTest = sqlContext.Tests.Where(x => x.ID > 0).ToList();
sqlContext.Tests.Add(new Test()
{
    Name = "name_" + DateTime.Now.Ticks.ToString(),
    Family = "family_" + DateTime.Now.Ticks.ToString(),
});
sqlContext.SaveChanges();
sqlTest = sqlContext.Tests.Where(x => x.ID > 0).ToList();

the first context works fine but the sqlcontext get the below exception:

An unhandled exception of type 'System.NullReferenceException' occurred in EntityFramework.dll Additional information: Object reference not set to an instance of an object

but if I remove the DbConfigurationType decoration then the second sqlContext works fine the first one give below exception:

An unhandled exception of type 'System.Data.SqlClient.SqlException' occurred in EntityFramework.dll Additional information: Login failed for user 'root'.

I know that is because of DbConfigurationType which can define in application start or decorate on Context or defined in config file....

but how can I have this (multiple connectionstrings and one context)?