using parallel batteries for starting

i have an older outboard that often takes awhile to get started, and sometimes drains a battery pretty low. my question is, if i have two batteries that arent in the best shape ( have 12 volts but only 8 on a load) and hook them up in parallel, will this help me get the last leg out of the batteries?

another question, i just got a "used" but less than a year old trolling motor battery. can i use another deep cycle battery i have with the new one in a 24 volt system. the other battery i have is older, not the same size, brand, or cranking amps.
i will be charging the batteries with a 4 bank on board charger

Yes to both your questions. However, you won't get as long of a run time with the troller as you would with 2 identical batteries. The smaller one is going to drain quicker than the bigger one giving you less power.

"The smaller one is going to drain quicker than the bigger one giving you less power."
but it wont hurt the batteries will it? or the trolling motor?
if i have one good battery and one bad battery, will it cause the good battery to "charge" the bad battery. causing the good battery to go bad. i know it will if wired parallel, but will it if its wired in a series?

If you connect a junk battery in series with a good one, more power will be drawn from the stronger battery than would be drawn if you had two good ones. It will be discharged more deeply every time you use it. That will mean a much shorter life for the new battery.

Don't do it! Your "bad" battery will suck the life out of the good one, shortening your run time AND the life of the good battery. Hooking up a couple that are different amperage doesn't cause a problem as long as both hold a good charge, however, you'll lose the lower amperage battery first and then it'll begin to draw down the other one, which negates the benefit of having the higher amperage battery. Best bet is "marry" up your batteries to get the best results, making sure to keep them charged at all times. A discharged battery begins to deteriorate very quickly and will be damaged in a relatively short time if left that way. If you're storing for the winter, check the charge a couple of times to make sure you haven't lost any charge and you'll be good to go as soon as the weather turns in the spring.

"God made men, Colt made men equal, Winchester made men equal at greater distance."

well, i then newer battery is a everstart (i think, whatever walmart carries) deep cycle, its the bigger or biggest version. the only good battery i have is also a everstart, says it is a multipurpose (starting, deep cycle) but it is the smaller version of the newer one. so as long as the older battery doesnt die out on me then ill be fine? it will just shorten my run time to the smaller batteries run time...which i dont really care about because i wont be out all day, or even have the day for that matter, i just like having 24 volts to get the boat moving, especially when its really windy.