You can certainly connect lead acid batteries in parallel. Many RVs, boats, and home photo voltaic systems do exactly this. Precautions must be taken to ensure that they are charged and discharged at the same rate, and that the batteries have similar internal resistances over their cycles and their life spans.
Two ES17-12 batteries connected properly in parallel will give you a stable 12 volt supply.
However, rather than asking what you can do with batteries that you have, determine what you need out of a battery and then use one with those specifications.
The spec sheet for the competition batteries is available at
http://www.mkbattery.com/images/ES17-12.pdf
This battery is 12 volts. Would a 7.2v or 24v power source let your project run more efficiently? This battery is sealed lead acid. Is that going to be robust enough to be safe for your application, or is it overkill? The battery can supply different very different amounts of energy at different rates, and depending on how low the voltage can drop in your application. Would two batteries supply as much as you really do need?
You must identify what you need to do before you can evaluate any solution.
~Phil