group by id and random sample by id from two tables in big query sql

I have a 2 tables with the same structure:

table1

id text        var  
1  "bla bla"   100
1  "blabla1"    30
2  "qweweqty"    0
2    etc...
7
3
3
1
..
100

table2

id text        var  
101 "bla bla"   10
101  "bla1"      60
101  "bla"    5
103    etc...
102
103
102
110
..
200

I want to randomly sample data from table1 and table2 based on id. so basically, sample every observation for a random sample of ids from table1 and and every observation from random sample of ids from table2 so that 50 ids are from table 1 and 50 are from table 2 . any idea on how to do this on big query SQL?