This shouldn't be a problem as you just need to put in terms of the iteration number and evaluate as usual.

For example for the second order RK method you would iterate the two equations as follows. First I'll write your differential equations in the general form:

and

so and need to be stepped simultaneously as follows:

then

and of course .

So for your problem you start with at and the following evaluations:

so then

and so on!

Well, hope that helps!