J_aco,
You would not normally set up a loan split and then deposit the additional funds into the offset, you would leave them in the split loan for redraw for when you made the investment.
I would be cautious about borrowing to the limit each time, in some instances it should be done, other cases perhaps not the best strategy. I agree that all investors should have a healthy safety net at all times as you do not know what is around the corner, but I have seen clients caught in the past of not qualifying for the next loan with another lender due to their LOC limits already established, even though not necessarily drawn down.