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Self-answered questions can be OK here.  Those are best when they are canonical questions on topics likely to come up.  Some examples where I have done this are:<ul>

<li><a href="https://electrical.codidact.com/questions/276305">What is a bootloader, and how would I develop one?</a>

<li><a href="https://electrical.codidact.com/questions/276136">How to calculate pullup resistor value for pushbutton?</a>

</ul>

In all such cases, the objective was to get a basic issue out there with an accessible and understandable answer.  This was also to provide a target for closing future questions as duplicates, on topics that experience has shown (elsewhere) are common duplicates.

Whether a question is self-answered or not, the question must be well written.  It has to be clear, reasonably answerable, and the answers not be opinions or devolve to a popularity contest.

This is where you failed.  Your answer was vague, included pontificating in the question, and left the reader wondering what was really being asked.  If the site had been more mature with multiple daily questions, I would have just closed it and not looked back.  In this case (a mistake, in hindsight) I tried to answer it, hoping it would get some interest and eventually result in useful content.

Since I can hear you getting ready to argue the point, here is your original question with commentary:

<blockquote>Title: Is it possible for one transistor to switch between two loads?</blockquote>

That's not so bad.  This made it initially seem like a reasonable question.

<blockquote>The output collector-emitter part of a transistor can be thought of as a 2-terminal SPST switch controlled by the input base-emitter voltage or base current.</blockquote>

This is lecturing, not asking.  Some amount of context-setting is important, but it sounds like you're trying to make a point instead of really wanting to know something.

<blockquote>So this <i>transistor switch</i> can control only a single collector load.</blockquote>

Here we see a big problem with lecturing in an answer.  This is just plain wrong.  It now takes extra work to answer, because this incorrect assumption first has to be dispelled before an answer can be supplied.

<blockquote>Is it possible to make the transistor act as a 3-terminal SPDT switch?</blockquote>

Whoa, how did we get to this mess from wanting to switch 2 loads with one transistor?  I really should have just close-hammered the question at this point.

<blockquote>If so, one transistor will be able to control two loads.</blockquote>

More annoying pontificating.

<hr>

The real problem was that your question failed the requirements by not being clear, concise, and answerable with answers that can be judged right or wrong.

Worse yet, it eventually came out that you were just trolling for a particular answer.  You got answers to your vague question, then would refine it when you didn't get the answer you wanted.  You were wasting the time of anyone answering thinking you had a genuine question and were trying to help you understand the issue.

Having to go back and forth on a question is annoying enough, but it's really irritating when it is eventually revealed the asker wasn't trying to learn anything.

<hr>

So what to do?

If you think you have a good way of explaining something others are likely to bump into, it's OK to ask a specific question and then answer it.  <b>However</b>:<ul>

<li>The question must be clear and to the point.

<li>You should provide your answer quickly.  It's really rude to ask other people for favor, have them spend time helping you, and then reveal you were just trolling for answers to contrast against your (what you think is) brilliant take on the subject.

I don't think this site has a button that lets you add an answer before the question is posted.  That means you should compose your answer off line, and copy and paste it as quickly as possible after asking the question.  Avoid jerking around the volunteers.

<li>The answer should be well written.  Canonical questions don't work well with sloppily written or confusing answers.

</ul>

The point you were apparently trying to make was that a SPST switch can be used to switch between two separate loads when those loads have different threshold voltages for drawing meaningful current, like two LEDs with different forward voltages.  That by itself is OK, and could have made a passable self-answered question if only it had been executed properly.  A sample question wording might be:

<i>I have two LEDs with different forward voltages.  Can I use a SPST switch to control which one is lit with the other off?  If so, how?</i>

Notice that this is clear, to the point, and specifies what the result should be.  Ideally by the time others see this question, they also see your answer.  They therefore won't waste time trying to help you.  However, they have a clear spec to provide alternative answers for.

Another option is to write a Paper, not a self-answered question at all.  We hold papers to a higher standard, and they will be voted on by the users.  Read the help on the Papers category carefully before deciding whether to write a paper or not.