What are the advantages/disadvantages?
 

Using 2 receivers in a 1/3 scale plane?
Using 6v instead of 4.8?

What are the advantages/disadvantages?
 

Advantages with two receivers if one dies you still have control of at least some part of the plane. Disadvantages, more power consumtion and more failure points because of complex electronics. 6v's depends on the servo's you're using, some are designed to work better on 6v than others. I'd say 6 volt regulated as the best benefits vs downfalls, as a full 6v pack is actually over 7 volts and that just seems dangerously high for a servo motor to me.

What are the advantages/disadvantages?
 

Dual receivers
With dual receivers you have some control redundancy if one fails (very rare). It also splits the power requirements between two receivers so you have some load sharing in high demand current airplanes. The disadvantages are that you now have an additional failure point, a higher weight and wing loading, and using dual receivers reduces overall range of both receivers just a bit. Unless you are running say 12 or 14 servos, you are better off with a single receiver. Receivers rarely fail, and are very reliable. Typically, most people start to use dual receivers in 35-40% or larger airplanes.

6V vs 4.8V
With 6v systems, you have more headroom against voltage depression, as the voltage delivered at the servo and receiver is higher. With a 4.8v system, you will not see 4.8v at the servo because of the wiring loss. With 6v, you will see a voltage above 4.8v, but below 6v. As you place a load on your batteries, the voltage will drop accordingly. On larger airplanes, running a 4.8v system could easily drop below the receivers voltage requirements during hard maneuvers. This would cause the receiver to start clipping. With a 6v setup, you have an additional 1.2v from the start, so it eliminates that problem for the most part. The servos will also operate a bit faster, and provide more torque because of the higher voltage. Most all manufacturers recommend using 4.8v or 6v setups for their servos.

The disadvantage is that you loose about 20% usable capacity compared to a 4.8v system. So you must increase your capacity by about 20% when you switch from a 4.8v system to a 6v system. Ie, if you were using a 4.8v 1000mah pack, you should use a 1200mah pack if you switch to 6v.

You should search for 6v vs 4.8v and dual receivers. There are lost of posts which detail all the trade offs involved. The answers are there for the taking.