Make table with two record unique

this is very simple.
this case call composite primary key. you can google for this.

create new table 
tbl_user_product_relation 
sUsername Primary Key char(8)
sProductID primary key int(11)
sDatetime timestamp current_timestamp

try to add entry directly from php myadmin
sUsername : tony
sProducID: 1

sUsername: tony
sProductID: 2

sUsername: joe
sProductID: 1

all above will successfull. and below will not allowed

sUsername: tony
sProductID: 1


this is what do you want, right?

in PHP you need to check this 
select * from tbl_user_product_relation where sUsername=? and sProductID=?
if num_rows() == 1 Insert
else echo 'duplicate'

NOTE:

hei.. what about my sID?

you dont need that. What you know about table is: every table must have id as primary key and auto_increment
Nooooo... that's totally wrong.
and if you pretend to not remove sID, you will have 3 composite primary key, and you will always have duplicate possibility,

because:
sID sUsername sProductID
1 tony 1
2 tony 1
3 tony 1
100 tony 1

is different