Now let's look at what you propose. You have a 5 KW generator and a 10 KW generator for openers. You closely monitor the demand on the load. If the load exceeds 5 KW you want to switch the load to a 10 KW generator. This looks simple and good on paper but has problems. First, you don't want to run a 5 KW generator under full load constantly, you don't want an instant transfer at 5 KW of load. The 5 KW could just be a spike in demand. You want a delay in there. Maybe like if the load demand exceeds 4.5 KW for several seconds then transfer. Lacking a delay the transfer will freak out (OK get confused) and bang back and forth. Things will get ugly! Additionally if the load is phase sensitive bad things will happen.
Now if the load exceeds 10 KW your options are to bring a 15 KW generator online or combine the 5 KW and 10 KW generators. Remember we are assuming all these generators are up and running. Actually if all the generators are up and running I would just have a single 15 KW generator and be done with it, however, moving along...
Someone mentioned the phasing. You may rest assured that if you have a 5 KW generator and decide to parallel a 10 KW generator lacking perfect sync and control really bad things will happen. You will see sparks, fire and smoke like you never dreamed of.
There are a few considerations in what you want to do or propose and you need to be aware of every one of them. That is what I am saying.
Ron