Usually, if you run multiple startups it is a clear signal that you are not committed to any of them, and is not going to look good for you. However, one reason to have multiple things going on, typically, is if you have a consulting company that is paying the bills necessary to launch your startup. Eventually, you will end up winding it down to focus on the startup full time, but if that's how you get the company off the ground, then it's a valid reason to have multiple businesses going on simultaneously.
If your current idea isn't working, you could definitely try a new idea, but Hiten suggests that does the two ideas in parallel is disservice to both you, and the company. If you think the old idea is not work, then stop working on it completely and go do the new one, or rethink the old idea, and figure out a way to make it successful.
Once a found moved on to a new idea, the old one is dead anyway, so there is no point in artificially keeping multiple businesses alive.
Doing multiple things is really hard, but previously successful entrepreneurs can potentially pull it off by starting companies, and then quickly assembling teams to run them.
Garrett Camp, at Expa, for example, has spun out a dozen startups, but he is not involved in the day-to-day. Instead, he has a significant amount of equity in each, and then assembles a team to run them.
Hiten Shah and Neil Patel were lucky enough to start Crazy Egg together, a company for which they were not able to raise venture funds. Even after moving onto another venture funded company, they have kept Crazy Egg around, putting in a part-time CEO to keep an eye on the basket. Many years later, the company is still operational and is bringing in revenue. You can make it work, if you insist on doing it your way.