I'm hoping this is an easy question but I can't figure it out for the life of
me.

I've simplified my scenario down to two variables, order quantity and order
frequency (represented by the number of days between orders). I want to
predict a customer's next order date and quantity based on past order
history. So I have two columns with historic information, one for the order
quantity the other showing the number of days since their last order.

If I try to use the forecast function I end up with a circular reference
because I'm trying to predict both variables. (ie I would have two forecast
functions in the last cell of each column, each function would want to know
the value of the other cell's forecast to make its own prediction).

Anyone know how I can do this? Here's sample data (note that because I'm
looking at the number of days since the last order, the first order does not
have a value for # of days, if this screws up your method feel free to drop
the first order's quantity from the analysis)

# of Days Quantity
2
14 2
14 2
21 2
14 2
28 2
21 2
35 2
14 2
14 2
14 2
14 2
14 2
14 2
28 2
14 2
4 2
10 2
7 4
21 2
14 2
6 2
15 2
10 6
32 10
145 2
1 2
15 2
21 2
7 2
11 2
15 2
9 4
13 2
15 2
14 2
14 2
14 2
7 2
26 2
20 3
10 2
7 1
11 2
17 2
14 2
14 2
21 2
21 2
20 2
36 2
14 2
14 2
21 2
10 2
11 2
14 2
14 2
14 2
14 2
21 2
14 2
7 2
14 2
21 2
11 2
3 2
4 2
17 2
14 2
14 2
10 2
32 2
41 4
27 4
9 8
21 4
6 4
15 4
19 4
21 4
43 4
35 4
40 4
24 4
63 4
x y

Need to predict both x and y...

If you know of any excel add in or third party software that would be a good
choice for this type of analysis I'm all ears (especially if it addresses
seasonality, variability and a bunch of other factors that throw predictions
off).

Thanks!!!!!