Nesting DataSourceLoops

I am trying to nest multiple data source loops. For instance, say I have a request with two inputs, Letter and Number. I define two DataSource steps, Letters and Numbers.

Letters:
a
b
c
d
e

Numbers:
1
2
3
4

Then I have my request, then the two data source loops. What I want is
a1
a2
a3
a4
b1
b2
b3
b4
...


what I am getting is
a1
a2
a3
a4
b4
c4
d4
e4

which makes sense, because they are running sequentially, not as nested. Can I do nested? Any help greatly appreciated.

David