2x 4g batches for 5g carboys??

Was thinking of doing a 9.5 gallon all grain nut brown ale? Then splitting it in half for the boil(s). Would then do different hops and schedules. Also would do different yeast in each batch. Then ferment each 4 gallon batch in my 5 gallon carboys at the same temperature for the same time. The thought is it would allow me to tune in a taste to my liking quicker. If I had had 3 gallon jugs I could just split a 5 gallon batch….. But I don’t. Ideas, suggestions…..

Simple recipes with simple hop addition ideas would be most welcome as they are easier to tweek.

Do you have a recipe that you are using already?

Do a little googling on SMaSH (Single Malt and Single Hop) batches.
Those are intended to give you a good idea of what different types of malts and hops taste like.

5 Gal batch is

2 Row 9#
Biscuit .25#
Choc malt .25#
Roasted Barley .5#

Fuggles 1 oz for 60 min batch 2: chinook 1oz for 60 min and Fuggles .5 oz for 15 min

still need thoughts on 2 different yeasts

To be clear, is your main reason for doing a 9.5G batch to reduce headspace in your carboys? I only ask because I've done a few batches in which I split 5 gallons into a pair of 5G carboys -- 2.5G in each -- and tested different yeasts in much the same way you are looking to. My understanding is that the CO2 produced during fermentation will fill the additional headspace, negating the oxygen issue.

I'm sure someone more experienced will chime in and correct me if I'm way off base!

yes! I have two 5 gallon carboys that I use for secondary fermentation. I thought I could ferment in them too but reduce the wort to about 4 gallons. I knew I could ferment 2.5 gallons in a 5 gallon carboy but why waste the extra space. If I'm going to make beer, I might as well make the max that will safely ferment. So I guess an 8 gallon batch to split between the two is more prudent. Also saw a SMaSH for Munich II and Tettnanger that looked interesting.