Well it is a gamble. Logic says the 6V batteries will probably age quicker than the 12V batteries (since they are already two years older) and when that happens they will begin to drain energy from the 12V batteries and reduce their effectiveness. You could install them using an isolation switch so you could switch from one set to the other as needed. A marine battery switch will also allow you to run them all at the same time or disconnect them all. Remember the 6V batteries neeed to be connected in series. Then if you visualize the two 6V batteries as a single 12V battery it needs to be hooked up in parralel to the other two 12V batteries.