Unless it's illegal, holding two full-time teaching positions isn't something bad by itself. Sure, it may be difficult to do both jobs to a satisfactory standard, but if you actually do, then what's the problem?
Some people will say it's not good for the work-life balance, but it's up to you to decide on your work-life balance, after all. And if you truly enjoy teaching, then why not to fill your life with what you enjoy?
Some will say you can't do both jobs to a satisfactory standard, but it's up to your employers to decide whether your teaching is good enough. Your teaching is good enough as long as they keep you employed.
And yes, you can do two full-time teaching jobs to a good standard. Preparation for lectures doesn't really take time if your have already taught your courses before. Likewise, grading doesn't really take time if you use automated grading systems in, e.g., Google Classroom. You say that your teaching jobs are online, so you don't need to spend your time commuting to work.
Lecturers are not hired to spend 40 hours a week. They are hired to deliver good teaching. How many hours per week they actually spend preparing for lectures and grading isn't and shouldn't be anyone's business.
If I saw someone's CV saying he or she has been holding two full-time jobs, I'd say, "Wow, you must be very smart and effective to do two full-time jobs to standards satisfactory to both employers!"
In short, don't be afraid to do what you want as long as it's legal, and be confident to explain things should people ask you questions about it.
UPDATE: To avoid a misunderstanding, the main point of my answer is that it is okay to to have two full-time teaching jobs if the quality of your teaching for each of the two jobs is good enough. But this is a very important if!
Of course, if your teaching is bad because of you having two jobs, this is not good. You will be eventually fired then, and before this happens, students will suffer. To accept two full-time teaching jobs, you must be sure you can do both jobs to standards acceptable by the employers!
My point is that having two full-time teaching jobs isn't bad per se; what matters is the quality of teaching for each of the two jobs. And if everyone is happy with your teaching, it's not their place to ask you what you do outside your job and whether you have a second job. The quality of your teaching is the ultimate criterion.
And I personally believe it is quite possible to do two full-time teaching jobs to a good standard. I am speaking from my own experience of studying at a university and making a lot of money by privately tutoring at the same time. The students whom I tutored were very happy with my tutoring and gladly paid me after each lesson. Had they been not satisfied with anything, they would have quickly found another tutor. My friends wondered how I managed to study and do so much tutoring at the same time. If you love what you are doing and exercise reasonable self-discipline, you can do much more than expected by other people!