Thanks everyone for the responses. I'm aware of the ideology on MarineHowTo.com regarding using deep cycles in lieu of a starter type battery. It was for that reason I chose not to install a starter battery when I first bought the boat. I figured I'd give it a try and see how it played out. It seemed fine until last weekend when my engine didn't start after about 10 seconds of cranking. When I attempted to start it a second time, there wasn't enough juice to crank it over. I checked the battery voltage and it was at 12.5. Fortunately, I had a portable battery pack that I put on it and was able to start the engine. My experience has been that in a perfect world, the deep cycles would probably be sufficient. However, I do not live in that world. It seems that if the engine doesn't start right away, the deep cycles don't have enough cranking amps for a second attempt. It is for that reason I've decided to install a starter specific battery and use the two deep cycles for the house bank.
If the batteries are healthy, a deep-cycle house
bank will easily out crank a single starting battery even at 50% SoC.. (this is not made up or theoretical but rather real world data). If the bank has been abused, has poor wiring, terminations etc. then that would be true for any type of battery. Keep in mind that most "deep-cycle" labeled G-24, 27, 29, 30 or 31 batteries have very similar construction to a cranking only battery only with thicker grids, better envelopes etc... When you place two or more house batteries in parallel the single start battery can't hold a candle to them. Here in the shop we started a large Yanmar over 40 times on a single deep-cycle battery with zero charging in-between. My guys finally got bored and stopped... If your deep cycle battery dies after 10 seconds of cranking, it had some major issues.
Heck the batteries for all our heavy equipment up North, a Cat D3 dozer (12V), Cat 320L excavator (24V), 40HP Kubota (12V) and a F550 dump body (12V) are all used marine deep-cycle batteries that still had life left in them when the owners upgraded to AGM, GEL or even LiFePO4. We've never once had a non start issue. We've had to crank the D3 quite a bit to bleed the system and the used DC batteries still pulled it off. The Kubota is the same exact engine used in boats. It has had a single used group 24 deep cycle battery in it for 3 years now and we run that tractor year round often starting in in below 0F temps.. In a starting only application I can get about 3-5 more years out of these batteries, and they're free.