What is the best way indexing foreign key from multiple table?

[Gilbert Consellado]

What is the better way of indexing the foreign key?

Create Table table3(
t3_id int not null auto_increment,
t1_id int not null,
t2_id int not null,
primary key (t3_id),
index IX_index (t1_id, t2_id), // this is my concern
constraint FK_t1 foreign key (t1_id)
reference table1(t1_id),
constraint FK_t2 foreign key (t2_id)
reference table2(t2_id));

or

Create Table table3(
t3_id int not null auto_increment,
t1_id int not null,
t2_id int not null,
primary key (t3_id),
index IX_t1 (t1_id), //this is my concern
index IX_t2 (t2_id), //and this
constraint FK_t1 foreign key (t1_id)
reference table1(t1_id),
constraint FK_t2 foreign key (t2_id)
reference table2(t2_id));

This is for innodb tables, I dont have a broad understanding how the btree works. But as far as I know, the first table will save the indexes on single area while the second is not, (correct me if I am wrong). And if it is what is the pros and cons if I will put it on a single index or multiple index?

Thanks.