You have your hands full, but you don't need me to tell you that... I do not believe I have seen any modification plans like you describe here. Perhaps someone else will answer. I am assuming you already have the Acorn lift installed, using it for your husband, so you just want to modify it? You are a step ahead of most of us if you at least have a lift installed for your home medical needs (they are expensive). However, if you do not already have one installed, then you might look at wheelchair lifts, they would consist of a platform.
If you cannot find plans and decide to attempt modifying an Acorn lift yourself, I might suggest calling Acorn first and explaining what you have in mind. I'm not an engineer, but it may be important to have the weight, especially a heavy weight, centered over the track, instead of cantilevered out to the side? I do not know how having a large weight off the side would affect the chair itself where it is attached to the track, or how it would affect the track where it is attached to your stairway. You probably want to be sure it's not going to damage the chair which you need for your husband. Wouldn't that be great if it would work, though?
I hope you can find a way to solve this question. To be honest, it has been a persistent and ongoing need here, both for big dogs, and for older owners, and for those of us in both situations. I might suggest a backup plan, which is to fence off the bottom of the steps (in some way that does not hamper the operation of the chairlift). People (including myself in this situation) have reported their dog barking at night when left downstairs, however at least in my case, my dog did eventually stop, though it was very frustratring (for both of us, no doubt) during the first couple of weeks when he barked a lot. It made it hard to sleep...but he did finally adjust to the "new normal".
I am sorry you are in a situation where you have to care for both a husband and a large dog (just as I am sorry for people with a newborn when their dog goes down). I have had to be honest after going through this myself with a dog I could not lift: remember that if you can't do it, however much you want to, that is not your fault.
I assume you have tried all the medications for hip and knee problems, etc.? Some people hesitate to use them, but in a difficult situation with a large dog going down, all I can say is, I got another 6 months of mobility for my old dog with anti-inflammatory medication. I still feel it was worth it.https://www.walmart.com/ip/Ktaxon-Baby- ... /245998283
I'm sorry you are asking a very clear and precise question, and this post has said everything except the information you are really needing. I hope you will find the plans you need.