Hi, Stephen. If you want to run two batteries in parallel to more or less double the current capacity, have a look at the "Battery Parallel Board" The P103 from Component-Shop.
You can buy it ready to use, or get it as a kit. Or you can buy the double Schottky diode from the internet and with a suitable heatsink build your own for the third of the price.🤞😊.
I tried to buy a kit a couple of years ago and they were out of stock so I downloaded their circuit diagram and instructions sheet (Components Shop) and etched a board and build my own. I needed the unit in a hurry to power my HDML and it's been working now for a long time with no problems 😀😀. Their unit is, I believe 🤔 rated at 20amps and only drops about 0.2 to 0.3volts whereas an ordinary diode/rectify drops 0.6volts..
The idea of the board is to stop one battery from discharging into the other. Good luck with your project, all the best Steve..👍

Thanks so much, Roger A1.

Of course, I'm not saying that paralleling is generally not recommended. In fact, I use it often.
I have 6-volt, 4-Ah AGM lead-acid batteries, and when I want to increase their range, I parallel them.
So far, I haven't found any 6-volt batteries with more than 5 Ah that are of an acceptable size and are suitable for my needs.
I preferred to make a general statement that is always valid and can be useful for everyone, because in this specific case, we don't know the voltage of the 9-Ah or 5-Ah battery, we don't know what type of battery they are (AGM, Lipo, NiMh, etc.), we don't know how large they are, or how much available space they have.
Maybe the title should suggest something?
There aren't even any photos.
However, it's likely that you're referring to NiMH batteries whose nominal cell voltage is known, and very common battery packs are 7.2 volts. These aren't essential questions, but they can help you conduct market research and understand your options.

I'm really glad you appreciated my contribution.

Great reply Alessandro, my thoughts too 😊

Hi Stephen
In planes I mostly use a single (LiPo) battery, and in boats 2 batteries in parallel as a rule (ideal for low-down balancing and not getting in the way of the drive train). As Bobby mentioned use batteries of the same caliber (e.g. 3s/4.8A in LiPo). They do not need to be identical as long as you don't run them down to zero - which you shouldn't anyway.
Something puzzles me: You speak of a 320 A speed controller. That sounds like top-notch racing - with much more stringent safety precautions (like identical batteries). But if you race your boats you would neither be using NiMH batteries nor asking this type of question . . . 😉

Stephen I regularly run two NiMhi 5 AH batteries in parallel. I always use identical batteries purchased at the same time. I would never use batteries of differing amp hour ratings in parallel. I have been unable to find any NiMHi battery rated above 6.5 AH.

Hi StephenT.
I assume you mean parallel connection logically.
I assume 5000 means 5000 mAh.
In series connection the voltages are added while the Ah remains the same. Of course we are talking about identical batteries.
In parallel connection the voltage remains the same while the Ah are added.
Regardless of the type of engine, if there are no particular needs that justify this choice, a single 9 Ah battery is better than two 5 Ah batteries.
In theory, with two 5 Ah batteries in parallel you would have an electrical charge of 10 Ah instead of 9 Ah (but you could see if there are 10 Ah batteries on the market), so two 5 Ah batteries would be better.
However, parallel connection has some problems. In parallel connection the batteries must be identical and with the same voltage before being connected.
You must ensure that they are detached from each other when not in use.
This is necessary because the more charged battery (more voltage) will charge the less charged one (less voltage), discharging itself and therefore there will be current flow.
Eventually they will discharge much faster than the normal self-discharge that occurs with open poles.
Two batteries in parallel should be imagined as a closed circuit with two ideal generators and two internal resistances.
In fact, the real generator can be represented as an ideal generator in series with a resistance (the internal resistance with a very low value).
I'll stop here but if you want other explanations to understand better there's no problem.
I would add that, in my opinion, inserting a diode between the batteries can only be a solution by considering the voltage drop of the diode itself (perhaps by choosing the right type).

In conclusion, as already mentioned, if you don't have geometric, space, bulk or other problems, use a single battery with a higher electrical charge.

P.S.
It may be useful to remember that Ah (Ampere Hour) is the unit of measurement of electric charge and not of current intensity (Ampere).
It is the same unit of measurement expressed in Coulomb. In fact there is a precise correspondence between Ah and Coulomb.
As everyone knows, units of measurement can be expressed in various equivalent terms. Ah is much more useful than Coulomb for a battery.
It may be useful to remember that the E.M.F. (Electromotive force, measured in Volts) of a battery, is not the same as the voltage of a battery.
In fact, the E.M.F. it is given by the sum between the battery voltage and the voltage drop on the Ri (internal resistance). The first is a nominal data while the second depends on the current flowing in the circuit.
It is easy to experience that the E.M.F. and rated voltage match, only in an open-circuit battery. In closed circuit the two voltages differ.
This happens because, according to Ohm's law, when current flows through a resistor (even if it has a low value), it creates a voltage drop.

Hi, if you connect them in series you will get double the voltage and the same amp hour rating.
If they are connected in parallel, the voltage remained the same but the amp hours will double.