Niko:

That is true. This can be understood from the Ms: Start Temperature of the Martensitic Transformation [°C] of steel. As per Kunitake,

Ms = 560.5 – 407.3 C – 37.8 Mn – 14.8 Cr – 19.5 Ni – 4.5 Mo – 7.3 Si – 20.5 Cu

As you can see the combined effect of Ni and Mo is to reduce the Ms by a larger amount than Mo alone. This increases the hardenability.

KUNITAKE, T. Prediction of Ac1, Ac3 and Ms Temperatures by Empirical Formulas. Heat Treating (Japan), 41, 2001, p. 164-168.

Yes it is known that molybdenum can increase the hardenability of steel, it forms carbides and may also provide secondary hardening during tempering. Nickel doesn't form carbides it dissolves into the host structure but as far as I remember when used together Ni and Mo have an interestingly increasing effect on hardenability. The following article also confirms the phenomenon:

https://www.google.com.tr/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CDwQFjAB&url=http%3A%2F%2Fwww.gkn.com%2Fhoeganaes%2Fmedia%2FTech%2520Library%2FInfluence%2520of%2520Chemical%2520Composition%2520and%2520Austenitizing%2520Temperature%2520on%2520Hardenability%2520of%2520PM%2520Steels.pdf&ei=hrEpU6O-IMikyAOstYDYDw&usg=AFQjCNEiBhN5Ax2aPh_MKokZeMF_jYIhzQ&sig2=Z4fkYjWY-c5gPNbR7-E29A&bvm=bv.62922401,d.bGQ&cad=rja

Nageswara and Sinan, thanks a lot for helpful and informative answers.

About Kunitake's equation, that doesn't require any co-operation between Ni and Mo, right? They only are the fourth most and the least effective elements on Ms temp, right? I mean in this sense any of the elements together has increased effect compared to the effect of either one alone.

About the work of Sokolowski and Lidsley, link from Sinan,

I wound it intresting, but it also raises some doubts about the Ni+Mo combined effect.

Like on page 4, second paragraph, where the 721 alloy with less Ni is stated to have "boost in hardenability" due to increased ammount of Mo and Mn. Though if other alloys are compared it seems that Ni may have some effect. But I can't find any remarks about the Ni-effect from the work... Sinan, did I miss something?

Niko:

That is correct. In most of the empirical relations in metallurgy, you would notice that all the effects of elements are shown separately. The equations do not have any combined effects term. That means the presence of individual element shows some activity but it will not influence the other element’s activity. That can be explained by the fact that each element forms carbide and promotes the necessary reaction to contribute to the hardening effect. For a comprehensive look at other empirical equations see Gorni’s Heat Treatment Handbook of formulas. A free version can be downloaded from here.

http://www.gorni.eng.br/e/Gorni_SFHTHandbook.pdf

Yes the article just mentiones the effect but doesn't have enough details on the phenomenon and it is true that Mn with Mo is much more effective in some compositions. I will also suggest you the following book, the related pages are available through google books (on pages 192 and 193 there is a comparison of different additives to Ni containing steels):

http://books.google.com.tr/books?id=_WkyJv9MOpEC&pg=PA192&lpg=PA192&dq=combined+effect+of+Ni+and+Mo+in+steels+hardenability&source=bl&ots=7BoIQi5MGu&sig=oiWcXXNQPZdEQeUxcr0J8Wiubs8&hl=tr&sa=X&ei=_8MqU8rnMYrbsgamyYHQBQ&ved=0CJEBEOgBMAk#v=onepage&q=combined%20effect%20of%20Ni%20and%20Mo%20in%20steels%20hardenability&f=false

And also in this paper the combined effect of Mo and Ni is further discussed:

https://www.google.com.tr/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&ved=0CE4QFjAD&url=http%3A%2F%2Fwww.metalpowder.com%2Fwp-content%2Fuploads%2F2013%2F07%2F2013-PM-Conf_Chaman-Lall_High-Strength-Nickel-Alloys-Rev-July-22.pdf&ei=_8MqU8rnMYrbsgamyYHQBQ&usg=AFQjCNGUTkc2OoNIjGZDRptpcbm3TTugMg&sig2=SYOhuoZG3erhj20awtU-wg&bvm=bv.62922401,d.Yms&cad=rja

The combined effects of alloying elements is a major discussion in steel industry, and there is still blind spots about the physics of the phenomena. I hope you can enlighten us all with your study.

Another approach may be to compare carbon equivalent (Ce) of Mn and Mo in steel. There are some emprical relationships in some books/articles to do that. Compare Ce with or w/o Mo; that may give an indication if combined additions of Mn and Mo is likely to increase the hardenability cmpared to Mn addition only.

I mean Ni not Mn.

Alloying with Mo increases hardenability due to formation of Mo rich M2C carbides.

Mo increases hardenability and strength at high temperatures. Nickel has effect on strengthening and toughening the ferrite phase and helps with increased toughness at low temperatures.

To see combined effect it is easiest to look at carbon equivalent of steel you are analysing.

You may find this link to be useful: 

http://www.materialsengineer.com/E-Alloying-Steels.htm

and also the references given below:

1) Clark D S and Varney W R,1962: Physical metallurgy for engineers (New York: Litton Educational Publishing Inc.) 2nd edn.

2)  American Society for Metals 1964: Metal handbook (Materials Park, Ohio: American Society for Metals) 8th edn, Vol. 2.

3) Avner S H 1974: Introduction to physical metallurgy (New York: McGraw-Hill Book Company) 2nd edn.

 4) Rollason E C: Metallurgy for engineers (London: Edward Arnold Publisher) 3rd edn., 1959

Best wishes.