I just spent 30 minutes writing a reply with links and lost the whole darn thing.
Current flow must be as close to identical as possible between the batteries to make sure they age the same and provide the capacity you think you have. Like 2 resistors piggybacked in a circuit; current flow depends on the total resistance from the point of entry to the point of exit. If the reistance is equal, each resistor carries an equal load. If the resistance is not, one "works harder" than the other.
1) External resitance can be managed by measuring cabling and making sure each "side" is excatly the same length.
2) So, the internal resistance of the battery is the key.
3) Internal resistance changes with manufacturer; battery type; capacity and use (number of charge/discharge cycles).
4) Use causes "sulphating" of the plates over time. The deeper the discharge per cycle, the more the plates get crystalized, the more the internal resistance of the battery goes up. Eventually the plates are so crusted over with sulphates that they will no longer hold a charge and the battery is "dead" and must be replaced. There are "De-Sulphaters" that will breath life into an old battery but they are never the same.
Rules of Thumb
Always change batteries in sets.
Always start out with two identical batteries.
Always recharge before battery capacity drops below 80%
High Current pulls reduce available capacity, (see attached charts)