Clearly the 6/1 isn't going to be able to generate 15kw just because the head has that capacity. That's ok, most of the time the load for the house is under 3kw, which the 6/1 should handle (at least, it would handle it on a 5kw head).
What I'm concerned about is if using the big head will result in significantly less electricty output per HP of motor, or difficulty starting the engine. For example, if the 15kw head making 3kw required 6.5hp from the engine whereas the 5kw head making 3kw only required 6hp. I don't know enough about generators to know if there is any drawback to having much more generator capacity than the engine can supply.
It sounds like this won't be a big deal and may actuall solve some issues which is great, because I'd rather buy the 15kw head.
Regarding driving a single head with two engines. If I had a way to measure if an engine was pushing or being pulled I could use some simple electronics to adjust the fuel to balance the load.
I could try putting idler wheels on the belts and a strain guage on the idler wheel mounts. When the engine is making power one side of the belt will be under tension, when it is being driven the other side will be under tension (to a lesser degree I presume). Electronically monitoring the belt tension via the strain guage on the idler would provide the information necessary to adjust the fuel to the engines to balance the load. I hesitate to invent something like this until I learn more about the usual solutions. I hate to complicate fuel control with electronics if a more reliable and cheap solution is available.
My current plan is to get a large battery backed UPS system (probably salvage from a datacenter) to carry the critical loads. The battery bank will be large enough to wait a few minutes for grid power to come back (its atypical for the power to be out longer than a few seconds, but it has been down for as long as two days). Once this part of the system is working I'll integrate a 6/1 and 15kw head between the UPS and (probably manual) transfer switch. The generator will come up and the UPS will go offline and recharge, allowing the generator to run everything. I'll manually ensure that the big loads stay off since the 6/1 won't have the capacity to handle them. Finally, I'll consider integrating a 12/1 so that I can use the big loads.
I suspect I'll find that I don't need the 12/1 at all because it will only be necessary for running the AC, dryer and ovens. All of this I can do without during an on-grid power outage. When I move off-grid I'll buy gas appliances and won't need AC at all (planning for earth sheltered concrete dome construction).
The purpose of the project is to learn how to run these things so I can move off-grid eventually, and also to give me some really cool toys to work with
Anyway, sorry to be so long-winded and thanks for the input!