Fail reasoning fails pretty hard.
BO earns 20% more xp than support.
If supports are to gain more xp than  the BO, you'd have to be using ships that are at least 32 lvl's lower than the BO, while the supports still gain full xp. That only happens if you don't rotate sailors, but leave some behind. So for most ppl supports will only fall farther behind the BO, even if you use the correct ship for them. Only using an engineer as PBB BO will allow supports to catch up to a BO, but not everyone likes using a BO with far too long guidelines.

If you plan on using a PBB, 2 BO's do not have to be brought up all that high, the 2nd BO probably doesn't need to be used all that much beyond PBB lvl to keep the supports in touch with your main BO ( standard 14 sailor crew). That's not all that much extra time for the 2nd BO.

Did... Did you read the words I used?... Fail fail is fail lol fail... -_-
Oh internet hipsters being ironic.

Saying it's "not all that much longer" is failed rhetoric, because what I suggested doesn't involve any extra time wasted on your second BO. Nothing ever falls behind, only the BO gets ahead because, well, it's the BO (plus gunners and hopefully scout, obviously). As many sailors are getting as much experience as is possible for their level. Period. 
Anything else is just preference.

It's just math. By the time you get to ships where you can use at least half of your crew (therefore at the point where you can factor in which way you want to do things from that point) if you go two BOs you're always one sailor short for both grinds (your BO) by default, as well as the less slots you're using because you're on low level ships twice.

At lowest levels (basically up until PCA) the loss of exp over your entire crew is less than you gain by having the extra support slot(s) from having one crew. It's then at PCA+ (10 supports if using same gunners, which being ahead can also be swapped depending on if AAing vs main guns or whatever you're doing) where you're at a perfect time to catch up to BO when the supports are all already close to each other. The entire time you're lvling more guys at the same time. 

Therefore, with PCA you're leveling 10 crewmen (all of your supports+1) to catch up to the BO which is by default ahead either way. Instead of grinding 6~ sailors including a useless BO (assuming you don't have an SS/CV crew waiting for it). 

And I'm assuming you meant far too short of guidelines with an Eng BO. Scouts can have enough potential to serve as fine Premium Ship BOs if you're concerned about it. 

I'm just detailing what's faster. Your preference is your preference. If fastest was also the simplest then everyone would only grind one way.

Well, going only one BO and rotating crew around makes you use that lower ships class for a longer time, maybe also twice as long as running with one BO through.

But after all I must say: I don't see a need for a second BO either if you only want to go down one BB/CV line. Get your full crew at the start and let them rotate around on any ships. As said use only ships of level matching your supporters. Be happy your BO and gunners run a bit ahead cause that means seeing shells and having more acceptable spreads on the ship's you are using.
You won't have that spread problems when making a second BO, but you'll have to spend money in that second BO just for beeing able to see shells landing and having guideline length.

I would only use a second BO if
- theres a second BB line for that nation and choosing those different lines
- you want to play a full set of FW or BW CV.

If you have premium the first 15~ levels it won't matter what ship you're in. Just put them on something that has a lot of slots (I myself use Moltke till lvl 15, then start again at say lvl 37-40. They can catch up to your BO even faster that way, if you're willing to buy a PCA). After that, say you're US, you get to Atlanta (lvling 5 + BO and Gunners) in really a couple of hours, where you can level everything through the first few levels of the game quickly. Basically, even if you're level 60 and your supports are somehow 40, that's a small smidge of all the experience you'll get over the course of your grind, so worrying that it takes an extra even fifty games on one ship isn't even palpable. 

However, if you think about it you can't say you're spending longer on the lower level ships because you'd still have to use that ship twice :P. And I assure you, it's very nice to always have above ship-level gunners ^.^ even just a few levels makes a noticable difference.