to start you should install Voukoder plugin, it speeds up encoding alot, if you still have much cpu even after then try another instance. I don't know if you can have 2 voukoder instances going at once though, although as that interests me i'll try now
I tried to vegas's both running voukoder encoder encoding x264, they work fine if you wanted to do that. Big speed difference with just a single instance of vegas Magix AVC - 48fps vs Voukoder x264 72fps . This is not entirely apples to apples. as I chose default settings for magix avc, but I chose the 'good recommended' setting for x264.
I would not worry about the quality, the quality is dependent on your used render configuration, not on how long it takes. I can imagine that rendering simultaneous will be faster than in sequence, as your system is more stressed, less chance there is idling between threads. Parallelism is a difficult subject, but is ultimately more efficant than sequential use (or nobody would use multi-core CPU's nor videocards with thousands of core units).
There is NO quality loss rendering multiple instances of Vegas & I do it all the time so my clients can receive a 4K intermediate, 4K HEVC, 1080p MP4, etc. rendered from the original source files.
On my 9900K / VEGA 64 workstation, (1) 2 hour 4K project takes about 2 hours to render to the same intermediate. If I open another instance of Vegas & render it to 4K HEVC, both finish in about 2.5-3.0 hours. If I open a 3rd instance & render to 1080p MP4, all (3) renders finish in 3-4 hours - depending on the complexity of the project. With (3) renders going, my CPU is maxed-out, but more importantly, so is my GPU... Therefore, IMO, for the same money, it is better to have (2) fast workstations (with their own GPUs) vs. (1) high-end 32-core AMD 3970X CPU, etc. that is still limited to having just (1) GPU...
I have noticed that when I render two files at the same time, it takes 6 hours. But when I render them separately they take 4 + 4 hours, 8 hours in total.
This made me think that the final quality was not the same. What do you think?
There are delays between rendering and encoding. It looks like less delay when using voukoder (x264) then built in encoders. this creates higher cpu and gpu use, and faster encode. If you are using vegaspro17, some people complain about it not using much cpu, that would benefit from 2 instances of vegas encoding at same time