public class TestModel
{
    public string A { get; set; }
    public string B { get; set; }
    public bool C { get; set; }
    public bool D { get; set; }
}

var model1 = new TestModel
{
    A = "a",
    B = "b",
    C = true,
    D = false
}

var model1 = new TestModel
{
    A = null,
    B = "b",
    C = false,
    D = true
}

var mergedModel = new TestModel
{
    A = "a",
    B = "b",
    C = true,
    D = true
}

Mapper.Initialize(cfg =>
{
    cfg.CreateMap<TestModel, TestModel>()
        .ForAllMembers(options =>
        {
            options.Condition((source, destination, member) => member != null);
        });
});

var mergedModel = Mapper.Map(model1, model2);