Combine the datas and find outliers or do not combine the datas.. HELP

I compared the stock returns between Australia and US over the period of 1991 and 1998

I found the summary measures(eg/min,max,Q1,Q2,Q3,coneficient of variation, stdv ect...)

I found the ourtlier for australian data and Us data seperately. There were few outliers for Australian and one outlier for US.

The question was If someone say"why dont we combine the two sets and then determine the outlier" and Ohter says" I'am not sure, I think it is not a good idea to combine"

Briefly write how you I response for these comments....

I have no idea whether i should combine the datas or not and i need a reason as well

Thank you for helping me =)