nah, you've pretty much got it pinned there Timmonds....
HOWEVER...
There are some actual "technical" bonuses to running a twin muffler system, aside from the fact they look horn....
Yes, splitting the pipes will slow the flow, so to speak....
However, by running two mufflers that are shorter than and of smaller diameter than your standard single muffler pittie zorst, it will be quieter, have a deeper exhaust note, and improve to some effect, bottom end power, as it will slightly increase backpressure through the system... similar to the tail cone in a two stroke expansion chamber, or the "powerbomb" effect of some thumper pipes, that run a small "bulge" in the middle of the header, eg 06 Yamaha YZF's and some of the older FMF and Big Gun thumper pipes... the idea is that it sends sound waves or pulses back down the pipe...
In a two stroke, it creates backpressure to "clear" the ports and transfer air/fuel mixture through the transfer ports, in a thumper, it slows the exhaust flow, creating backpressure, basically so that the engine isnt just "foofing" away in the bottom end like a generator....
Gone are the days where a good thumper pipe is just a drain pipe with fibreglass wrapping in it... have a look at an old XR600 pipe, then compare the diameter to a late model Jap MX thumper pipe.. muffler diameter is roughly half what they used to be.... dont need the outright flow anymore....
so in essence, with the right cam, head and valve work, carbie etc, you will be able to tune an increase in power using a twin muffler set up... you will also be able to do this with a single muffler zorst, but it will be louder, and wont have the wank factor, unless it's one of these $400 yoshi or DrD or FMF LXR style pipes...
On it's own, the only difference you will notice in using the twin muffler set up, is that it will be quieter, and have a different exhaust note.. those that have used them, have reported a much deeper tone to the zorst note, and not as crackly....
Plus they look better in twos... like tits... and bundy cans....