In the RV (Motor home) world, its common to join or link leisure batteries together in parallel to obtain a much bigger storage capacity.
They always recommend that the batteries are the same size, make and age. They then use a split charge relay on the vehicle to power the bank as one unit. If you mismatch leisure batteries, they apparently don't work well. You must never link a car (starter) battery and a leisure battery together in a battery bank.
The idea of the split charge relay is to separate the van battery from the leisure batteries so the van battery will not be discharged when using power from the leisure battery. The only time when the batteries are linked together is when the engine is running and charging the batteries. Preference is given to the van battery when starting the engine when using a VSR (voltage sensing relay or intelligent relay ) and only after that is the leisure battery charged.
Of course, there is nothing stopping you from powering your hose reel from a different battery which could even be a big starter battery (not leisure) but you would fit another VSR to charge that separately. You would not link the batteries together.
The most popular battery for twin operators with 2 pumps is a 110 amph leisure battery. A single operator can use an 85 amph battery quite successfully.
Leisure batteries used in RV's aren't usually much bigger than 135 amph. Its not uncommon to see an RV with 3 or more batteries joined together as one large battery bank. Anything bigger than that has to have a fit for purpose storage cabinet. Its easier, more convenient and space efficient to tuck smaller batteries into small unused spaces.
A Durite VSR is bi directional. That means that if you put the leisure battery on charge whilst its still connected up , your battery charger will also top up the van battery. Adding another vsr and another battery will mean that the poor old battery charger will end up trying the charge everything. This may cause it to fail prematurely.
Putting a small on/off switch on the earth wire of the Durite will switch it off and stop it working.