Electrophilic substitution on disubstituted benzene rings
Electrophilic Substitution of Disubstituted Benzene Rings

When a benzene ring has two substituents, each substituent exerts an effect on subsequent substitution reactions.


  Electrophilic substitution of a disubstituted benzene ring is governed
  by the same resonance and inductive effects that affect
  monosubstituted rings. The only difference is that it’s necessary to
  consider the additive effects of two different groups.



If the directing effects of the two groups reinforce each other, the situation is straightforward. In p-nitrotoluene, for example, both the methyl and the nitro group direct further substitution to the same position (ortho to the methyl and meta to the nitro). A single product is thus formed on electrophilic substitution.




From above, the methyl and the nitro group direct further substitution to the same position (ortho to the methyl and meta to the nitro). This is similar case with chlorine.


If the directing effects of the two groups oppose each other, the more powerful activating group has the dominant influence, but mixtures of products are often formed.
Further substitution rarely occurs between the two groups in a meta-disubstituted compound because this site is too hindered. Aromatic rings with three adjacent substituents must therefore be prepared by some other route, such as by substitution of an ortho-disubstituted compound.


You may also wish to review this table showing positions where common substituents direct:



Basing on the above explanations,  it is easy to predict the products of  electrophilic substitution of disubstituted benzene ring, the only thing you have to know is the effect of each substituent whether it is activating or deactivating. In some cases, observe the positions of the substituents as point 3 mentions steric effects may come into play and substitution may not occur.

On the second note, concerning diazonium ion, I think you have to look at measured dipole moments, experimental data etc I doubt if there is a way of telling whether a substituent is strongly deactivating by merely looking at it



After explaining concepts now lets see what the answer for the question in comments will look like:



Reasons


the directing effects are opposing (nitro is meta while methyl is both ortho and para directing) but the methyl is more activating so we have a mixture:


meta to nitro and methyl
ortho to the methyl but para to the nitro



In this scenario since position 3 is not deactivated by nitro group hence there is also a possibility of a mixture here, also take note of how the nitro group is strongly deactivating at position 4 (more additive effect)

Hope this helps

References 


McMurry Organic chemistry 
Electrophilic Substitution of Disubstituted Benzene Rings