Fusing + and - is common in some situations, such as for example HAM radios used in a vehicle where the +/- cables of the radio connect directly to the battery.
The reasoning is, that if the radio is mounted so its chassis is grounded say in the cab and then a main battery cable is disconnected, the radio ground wire now becomes the ground path for the vehicle between the body (via the radio chassis and radio's ground wire) and the battery and other loads (like the starter motor...) will push high current through that path.
With your tek, the dual fusing may be for a similar situation or combination of possible mis-wires (or reverse polarity connection of your battery...)
cheers,
george.