Old and new batery with load !!

I don't think there is a problem with it.  In fact, I think it is a fair way to extend the life of older, weaker batteries by giving them a strong buddy to prevent from over-draining.  I don't think the type of load is particularly important.  As long as they are in parallel and not in series, the newer, stronger battery will charge the older, weaker one if its voltage drops quicker.  Overall, your capacity will be bigger than if you throw away the old one and only use the new one, so there's no use doing that.  Your loads shouldn't notice any difference. 

I've been using a 550 Ah bank of Rolls Surrette batteries for five years, and added a second identical bank three years after I bought the first one.  These are the core of my off-grid house electrical system.  The older bank does tend to hold slightly less charge, and the newer one keeps it from draining as fast as it otherwise would.  These batteries are expected to last 15-20 years and I imagine I will probably add a third identical bank in the next few years and eventually start rotating them out as the first one nears its life expectancy.  Assuming of course we haven't converted to hydrogen or something by then.