"How if I take combination (A+B) of these two extracts to study its effect in diabetic complications."

Why not!

If you want useful advice, provide details.

For example, how do you define and measure anti-diabetic and antioxidant activities! 

Yes, it is always good when you do bio assay to also study the synergistic effects of compounds. Remember, plants always contain a mixture of compounds and they sometimes work in synergy to one another. You can develop combining ratios and test them for antioxidant and anti diabetes.  

Poly-herbal formulation is common in disease treatment.

I find it a good idea on one condition: to identify components of each extract and to try in your research many ratios. Then, you can recommend the best ratio of combination regarding both effects (blood glucose lowering and antioxidant effects). Since it is well known that antioxidant properties are beneficial in prevention of diabetes complications, I believe that combination of A and B in your research will be of value.

I also advise to refer to similar researches in which two herbal extracts from the same botanical families as A and B were used in order to be guided regarding dosage and approach of treatment. 

 The synergistic effect  is the main base for  drugs combination. So  If you have a plant extract (A) with antidiabetic& maximum antioxidant activity and  another plant extract (B),to take combination (A+B) of these two extracts for studying its effect in diabetic complications you must follow this steps:

1- you must investigate the antidiabetic effect in - vivo by inducing experimental diabetes in lab. animals like rats.

2-  Then classify the diabetic rats into several groups(usually 7) ; one of them left untreated as control negative, another treated with slandered antidiabetic drug, the third  gp. treated with a small dose of extract (A), the fourth gp. with double or larger dose (A);by means you can test many doses at least two doses.

3- Investigate the antidiabetic effect of extract(B) in also two doses at least 2.

4- In the last gp. make combination from (A+B) at the smallest  doses, then treated diabetic rats to detect the synergistic effect.

5- After 24 hrs. From the last dose, take Bl. samples and determine Bl.glucose, serum insulin levels, homa index, and antioxidant parameters in serum and tissues. 

Possible complications include:

Cardiovascular disease. ...

Nerve damage (neuropathy). ...

Kidney damage (nephropathy). ...

Eye damage (retinopathy). ...

Foot damage. ...

Skin conditions. ...

Hearing impairment. ...

Alzheimer's disease

I wish you all the best in combination of A and B for studying the effects on diabetes  complications, you will need many years to prove the activity of A+B 

It is not sure, if the combination will act in synergism with each other but chances are very high for them to act synergistically given both are plant based substances. the process described by Ms. Nehal in above reply is appropriate.