How about an identity column for each of the tables then?. 
Or do you have a need that the value of the orderno being used in TAB_A should NOT be the one used in OrderHeaderNo of TAB_B
TAB_A        
OrderNo    Col1    Col2
1    Text_1    Text_2
2    Text_1    Text_2
3    Text_1    Text_2
        
TAB_B        
OrderHeaderNo    Col1    Col2
4    Text_A    Text_2_B
5    Text_A    Text_2_B
6    Text_A    Text_2_B

If so then the question would arise as to what is your goal that you wish to accomplish?