How to optimize the life of a Lead Acid deep cycle battery?
1) Fully discharging a lead acid battery is bad for its life, going to only 50% is good, and will extend both batteries lives. Twice the installed capacity also gives you an emergency 100% reserve, if you're happy for that capital to be tied up permanently. 

Before you connect two batteries in parallel, they must (obviously?) be the exactly the same chemistry and voltage, no paralleling a wet 12v and an SLA 12v. They should also have the same voltage, equalise them with a resistor or a lamp if not.  It helps if they are the same age of battery.

2) Cycles are usually counted as a period of discharging, followed by a period of charging. It doesn't matter how deep this cycle it, it still counts as a 'cycle'. However, the battery life will be likely extended to thousands of these shallower cycles. It's more than word games, as battery life is non-linear with depth of discharge. If you start counting fractional cycles, then a 50% DoD might have to count as 0.2 of a cycle.

3) You might want to review that 14.2v figure for how you intend to charge the battery. It's a bit high for float charging SLAs, and a bit low for cyclic charging wet cells. Too low a charge voltage can lead to sulphation, which is another way to shorten the life of the battery. 12v as an endpoint will give you better lifetime than the 11.5v or even 11v that I've often seen quoted in battery literature.