A battery blender won't help with voltage sag in the same way that paralleling will. Paralleling basically combines the discharge rates of the two packs so it will cut sag in half under the same conditions. Both the blender and paralleling benefit from not discharging one of the batteries too low if your trip only requires on pack's worth of charge (two half charged packs vs a single fully discharged pack). The advantage of the blender is that you can connect the two batteries together without worrying about frying something, while paralleling requires you to make sure both packs are at the same voltage before connecting them.
Your next question should be, as 99t4 alludes to, is how you'll be charging the packs. The blender would allow you to charge each pack with it's own charger, while still connected to the blender (think of the blender as being a few diodes that prevent current from one pack flowing into the other. It's actually performed by MOSFETs in some/most blenders). Charging parallel pack may, or may not, be more complicated or dangerous, but you should be very comfortable that you know what you are doing or what could go wrong before attempting it, and have a well thought out routine to follow.
