If you put the jet pump on the discharge of the submersible it will cause the submersible to deliver more flow because it will be working against less pressure. Proper design of such a system should be based on knowledge of the pump GPM/head characteristic and the depth to water in the well.
You want to be sure that you don't operate the submersible beyond its range or you could overload it with too much flow.
Get the pump curve (or post the make and model number of the pump) so we can see the maximum rated flow for the pump. That maximum rated flow, often about 150% of the nameplate rating of the submersible pump, is the maximum that you should take out of the submersible.
As long as you don't exceed the GPM limit of the pump, the submersible/jet system will operate an a point where you will get increased flow at the same pressure or increased pressure at the same flow.
NOW TO THE RISKS!!!
If the flow is shut off or restricted you will probably blow up the jet pump!
Submersibles have a nasty characteristic of VERY HIGH PRESSURE when the flow is restricted. Add that to the boost from the jet pump and it will almost certainly fail the jet pump or the lines on the discharge of the jet pump, whichever is weakest.
You should have a relief valve on such a system that will discharge enough flow so the submersible will never approach shutoff pressure (less than 25% to 50%of it's flow rating).