Two-spacing is from the era of typewriters. Typerwriters had monospaced typefaces because of how they operated. One space didn't work well enough if the period is rendered center of the type block. To work visually for prose, you'd have had to shift it leftwards so it hugs up against the end of a sentence, leaving a wider gap afterwards. But then you'd end up with awkward decimal points for numbers.... so instead we decided to just use two spaces instead.

So, it's obviously fine to two-space if you're using a typewriter.

It's therefor also fine if you're writing in a monospace/fixed-width font.

But don't do it with proportional width fonts! They are designed with various hints in place to render appropriately as one should typeset them, adjusting spacing between various letters, and of course rendering periods and decimal points correctly, contextually!

HTML will also ignore extra spaces, accordingly. Word processors will either highlight consecutive spaces as a typo or even auto-correct as you type. So I guess not much worry here if it's a habit.