I have a SaleOrder with a Product set as:

And a task on which I create a timesheet and choose the Analytic Account pointing at my orderline. If I want to have another product of the same type it shows me

You can use only one product on timesheet within the same sale order. You should split your order to include only one contract based on time and material.

I want to be able to charge on the same invoice different services for ex:

1. Installing tracks 2hrs

2 Fixing cables 4hrs