Careful on this one...
With single function dual station mechanical controls, the usual set-up is for the upper to push-pull the lower to push-pull the throttle and transmission.
This turns out to be problematic with dual function controls because the calibration has to stay correct or one can ends up making high speed shifts and things like that. As a result, you won't find this type of system around very much anymore. You won't find it, for example, from SeaStar/Teleflex. But, I think someone does make one (I forget who).
To get dual station/dual function controls, one usually does one of two things: (a) Installs electronic controls, or (b) installs the controls using the solution that SeaStar/Teleflex does offer, which has a mechanical lever that actively and positively switches from one helm to the other, so each can be calibrated separately w.r.t. the transmission. It works, but, because one can only switch when both the upper and lower controls are in neutral, there is some inconvenience. Additionally, there can only be one upper-lower switch lever, not one per helm.
One thing to note, if you haven't seen it before, is that you can adjust throttle while in neutral by "pulling out" the lever and then rotating it. This can be done, regardless of which station is selected.
My boat uses this system. I have attached a .pdf with the catalog section and overview. See pages 3/5 and 4/5. I've also included the instructions for the various dual function parts, combined into one .pdf. And, I've included a picture of my switch lever, located net to the door by my lower helm. Notice the empty panel space beneath the know -- the lever and bracket extend into that space behind the panel.
Basically, you'd need a CH5300P dual function control for each helm, the switch lever, and the dual station units for both throttle and transmission, as well as the cable. I recently posted about trying to buy a BrandX knock-off instead of a Seastar/Teleflex replacement for my aging MT-2 controls (the predecessor to the MT-3, which is the predecessor to the CH5300P). The tolerances were bad and the fit and finish only acceptable. They could have been made to work. But, I decided to spend the money and get the brand-name units and quality.
Funny story. My boat has upper and lower helms and, when I was shopping for it, I notices that there were no throttle or transmission controls at the lower helm. The hole had been covered over with a "stuff tray". The MT-2 dual control unit was stowed. I asked the sales person about it. He told me the PO took it out because he didn't use it and wanted a place to put stuff and that all I'd need to do was to put it back in, if I wanted.
After buying the boat, I asked the PO about it. He told me it never worked during his ownership. It had been disconnected years ago. So, I called the PPO. He told me that the controls never, ever worked. He said that Hardin mostly made sailboats, which only have one helm. They put this system into an older sister of my boat, and there was some kind of accident and law suit involving the installation. So, Hardin quit installing the controls and left it to the dealer. The dealers, seeing the problem, left it to the owner. And, the owners never bothered, because the visibility isn't great from the lower helm. As for why both units looked sun-worn, at some point, when the cables were being replaced, they were swapped to make the one on the flybridge look new again.
I can't stand to see something disconnected or not working, so I reinstalled the controls. After researching it, I went with the dual station setup from Seastar/Teleflex. I put the control lever by the lower helm door. This makes it easy for me to come down from the flybridge and switch it over to quickly bump the throttles if I want to while tying up. And, for exactly that reason, I am glad I have it working. I don't need it very often. But, single handing in wind, on some days, am glad to have the lower controls working.
At any rate, I hope this is of some help to you.
. It does sound like your issues are cable related and new cables may be the cheapest fix.
