8500 W generator with L14-30 plug.. How to access full capacity for house?
Don't worry about it

Breakers have a certain latitude to them, and can run slightly above rating for a short time.  That provides for motor starting and short term overloads. 

You're not supposed to plan to overload anyway

You should be provisioning power for 125% of what you expect to actually draw.  So if you plan for 6800 VA of actual load, you should provision at least 8500 VA of supply.  So don't aim to draw more than 6800 VA.   

If you actually have 8500 VA of load in mind, then provision 125% of that or 10,625 VA. 

VA is not quite watts

Often loads don't use the entire sine wave.  The part they use is measured in watts.  But the entire sinewave must be generated, and that is measured in VA.  This can bite you. 

Anyway, it’s really hard to load the generator evenly

North American 120/240V comes in two poles of 120V each.  Each one has its own breaker trip, but they are common-trip, so the breaker will trip if either side exceeds 30A by enough margin or time. 

That means you must full balance your 120V loads to get full performance out of the generator. This is hard to do.  I would say impossible without ammeters on both legs, and even that is not telling you about the difference between watts and VA.  

If you find that issue objectionable, you can fix it by putting a 10 KVA transformer in front of the transfer switch. Now 120V loads will evenly load the generator, since the transformer is balancing.   

Never parallel

Electrical connections should never be paralleled, except in certain rare instances.  There are many reasons for it, including eddy current heating, but one problem is neutrals.  Current will flow as it pleases, while completely ignoring any breaker trip ratings.  If your breakers are 30/15 it will cheerfully flow 39/6 if it wants to, and trip your 30A breaker.  

But much worse is happening on the neutral.  With two competing neutral route, the same thing could happen and neutrals don't have breakers. So you're just setting the wires on fire at that point.  

Even when paralleling is done industrially, special equipment is used which deals with this important point! 

Suicide cords, never

I have done some janky things with power.  But making a cord with males on both ends is something I would never do.  No problem exists which requires that as a solution.  

Firstly you have the problem that you will inevitably have one end plugged into "live" while the other end is free to contact anything or anyone.   

Second, you have the problem that a suicide cord can do an end-run around your transfer switch/generator interlock.  That can be over to "utility" side while your suicide cord backfeeds the grid.  This defeats the purpose of having an interlock.