An alt account isn't necessarily a problem and many people do have one. Most of the time I see them as Pen Name for those who either wanna join a Free RP and not be bothered on their original account, or for 1x1 situations where they can remain anonymous (mostly for smut I gather).

In regards to joining your own roleplay, I can see that playing both positively and negatively depending on what the intention is.

Positive:
- You can play a character in your own RP without others guessing it's you. This could work well for mystery RPs where you, the GM, can work with yourself to kill off a character or make a character look like the villain and it will appear as a brilliantly planned moment with a player.
- You can use it as a combined account between GMs.
- It can make your RP feel bigger than what it is, eg: 5 players instead of 4.

Negative:
- Difficult to keep up with multiple accounts.
- Some players may not like the GM artificially inflating their own RP.
- A good GM can work with multiple characters anyway and it's their RP, so any characters should work just fine under the GMs name.

It's odd cause I'm looking at it and I don't really see any positives, but at the same time I don't see any negatives. The only thing it does is just use up time in remembering which character to post as and under which account, plus you'll need to ensure you post regularly or others will think you dropped.